diff --git a/command/013_config_upgrade.go b/command/013_config_upgrade.go
index 1c3a0a67b..850dfa525 100644
--- a/command/013_config_upgrade.go
+++ b/command/013_config_upgrade.go
@@ -2,15 +2,20 @@ package command
 
 import (
 	"fmt"
+	"io/ioutil"
 	"os"
-	"path/filepath"
+	"path"
+	"sort"
 	"strings"
-	"text/template"
 
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+	"github.com/hashicorp/hcl/v2/hclwrite"
+	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs"
-	"github.com/hashicorp/terraform/internal/initwd"
-	"github.com/hashicorp/terraform/moduledeps"
+	"github.com/hashicorp/terraform/internal/getproviders"
 	"github.com/hashicorp/terraform/tfdiags"
+	"github.com/zclconf/go-cty/cty"
 )
 
 // ZeroThirteenUpgradeCommand upgrades configuration files for a module
@@ -19,6 +24,9 @@ type ZeroThirteenUpgradeCommand struct {
 	Meta
 }
 
+// Warning diagnostic detail message used for JSON and override config files
+const skippedConfigurationFileWarning = "The %s configuration file %q was skipped, because %s files are assumed to be generated. The program that generated this file may need to be updated for changes to the configuration language."
+
 func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 	args = c.Meta.process(args)
 	flags := c.Meta.defaultFlagSet("0.13upgrade")
@@ -71,58 +79,412 @@ func (c *ZeroThirteenUpgradeCommand) Run(args []string) int {
 		return 1
 	}
 
-	// Early-load the config so that we can check provider dependencies
-	earlyConfig, earlyConfDiags := c.loadConfigEarly(dir)
-	if earlyConfDiags.HasErrors() {
+	// Set up the config loader and find all the config files
+	loader, err := c.initConfigLoader()
+	if err != nil {
+		diags = diags.Append(err)
+		c.showDiagnostics(diags)
+		return 1
+	}
+	parser := loader.Parser()
+	primary, overrides, hclDiags := parser.ConfigDirFiles(dir)
+	diags = diags.Append(hclDiags)
+	if diags.HasErrors() {
 		c.Ui.Error(strings.TrimSpace("Failed to load configuration"))
-		diags = diags.Append(earlyConfDiags)
 		c.showDiagnostics(diags)
 		return 1
 	}
 
-	{
-		// Before we go further, we'll check to make sure none of the modules
-		// in the configuration declare that they don't support this Terraform
-		// version, so we can produce a version-related error message rather
-		// than potentially-confusing downstream errors.
-		versionDiags := initwd.CheckCoreVersionRequirements(earlyConfig)
-		diags = diags.Append(versionDiags)
-		if versionDiags.HasErrors() {
-			c.showDiagnostics(diags)
-			return 1
+	// Load and parse all primary files
+	files := make(map[string]*configs.File)
+	for _, path := range primary {
+		// Skip JSON configuration files, because we can't rewrite them and
+		// they're probably generated anyway.
+		if strings.HasSuffix(strings.ToLower(path), ".json") {
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Warning,
+				"JSON configuration file was not rewritten",
+				fmt.Sprintf(
+					skippedConfigurationFileWarning,
+					"JSON",
+					path,
+					"JSON",
+				),
+			))
+			continue
+		}
+		file, fileDiags := parser.LoadConfigFile(path)
+		diags = diags.Append(fileDiags)
+		if file != nil {
+			files[path] = file
+		}
+	}
+	if diags.HasErrors() {
+		c.Ui.Error(strings.TrimSpace("Failed to load configuration"))
+		c.showDiagnostics(diags)
+		return 1
+	}
+
+	// It's not clear what the correct behaviour is for upgrading override
+	// files. For now, just log that we're ignoring the file.
+	for _, path := range overrides {
+		diags = diags.Append(tfdiags.Sourceless(
+			tfdiags.Warning,
+			"Override configuration file was not rewritten",
+			fmt.Sprintf(
+				skippedConfigurationFileWarning,
+				"override",
+				path,
+				"override",
+			),
+		))
+	}
+
+	// Build up a list of required providers, uniquely by local name
+	requiredProviders := make(map[string]*configs.RequiredProvider)
+	var rewritePaths []string
+
+	// Step 1: copy all explicit provider requirements across
+	for path, file := range files {
+		for _, rps := range file.RequiredProviders {
+			rewritePaths = append(rewritePaths, path)
+			for _, rp := range rps.RequiredProviders {
+				if previous, exist := requiredProviders[rp.Name]; exist {
+					diags = diags.Append(&hcl.Diagnostic{
+						Summary:  "Duplicate required provider configuration",
+						Detail:   fmt.Sprintf("Found duplicate required provider configuration for %q.Previously configured at %s", rp.Name, previous.DeclRange),
+						Severity: hcl.DiagWarning,
+						Context:  rps.DeclRange.Ptr(),
+						Subject:  rp.DeclRange.Ptr(),
+					})
+				} else {
+					// We're copying the struct here to ensure that any
+					// mutation does not affect the original, if we rewrite
+					// this file
+					requiredProviders[rp.Name] = &configs.RequiredProvider{
+						Name:        rp.Name,
+						Source:      rp.Source,
+						Type:        rp.Type,
+						Requirement: rp.Requirement,
+						DeclRange:   rp.DeclRange,
+					}
+				}
+			}
 		}
 	}
 
-	// Find the provider dependencies
-	configDeps, depsDiags := earlyConfig.ProviderDependencies()
-	if depsDiags.HasErrors() {
-		c.Ui.Error(strings.TrimSpace("Could not detect provider dependencies"))
-		diags = diags.Append(depsDiags)
-		c.showDiagnostics(diags)
-		return 1
+	for _, file := range files {
+		// Step 2: add missing provider requirements from provider blocks
+		for _, p := range file.ProviderConfigs {
+			// If no explicit provider configuration exists for the
+			// provider configuration's local name, add one with a legacy
+			// provider address.
+			if _, exist := requiredProviders[p.Name]; !exist {
+				requiredProviders[p.Name] = &configs.RequiredProvider{
+					Name: p.Name,
+				}
+			}
+		}
+
+		// Step 3: add missing provider requirements from resources
+		resources := [][]*configs.Resource{file.ManagedResources, file.DataResources}
+		for _, rs := range resources {
+			for _, r := range rs {
+				// Find the appropriate provider local name for this resource
+				var localName string
+
+				// If there's a provider config, use that to determine the
+				// local name. Otherwise use the implied provider local name
+				// based on the resource's address.
+				if r.ProviderConfigRef != nil {
+					localName = r.ProviderConfigRef.Name
+				} else {
+					localName = r.Addr().ImpliedProvider()
+				}
+
+				// If no explicit provider configuration exists for this local
+				// name, add one with a legacy provider address.
+				if _, exist := requiredProviders[localName]; !exist {
+					requiredProviders[localName] = &configs.RequiredProvider{
+						Name: localName,
+					}
+				}
+			}
+		}
 	}
 
-	// Detect source for each provider
-	providerSources, detectDiags := detectProviderSources(configDeps.Providers)
-	if detectDiags.HasErrors() {
-		c.Ui.Error(strings.TrimSpace("Unable to detect sources for providers"))
+	// We should now have a complete understanding of the provider requirements
+	// stated in the config.  If there are any providers, attempt to detect
+	// their sources, and rewrite the config.
+	if len(requiredProviders) > 0 {
+		detectDiags := c.detectProviderSources(requiredProviders)
 		diags = diags.Append(detectDiags)
-		c.showDiagnostics(diags)
-		return 1
-	}
+		if diags.HasErrors() {
+			c.Ui.Error("Unable to detect sources for providers")
+			c.showDiagnostics(diags)
+			return 1
+		}
 
-	if len(providerSources) == 0 {
-		c.Ui.Output("No non-default providers found. Your configuration is ready to use!")
-		return 0
-	}
+		// Default output filename is "providers.tf"
+		filename := path.Join(dir, "providers.tf")
 
-	// Generate the required providers configuration
-	genDiags := generateRequiredProviders(providerSources, dir)
-	diags = diags.Append(genDiags)
+		// Special case: if we only have one file with a required providers
+		// block, output to that file instead.
+		if len(rewritePaths) == 1 {
+			filename = rewritePaths[0]
+		}
+
+		// Remove the output file from the list of paths we want to rewrite
+		// later. Otherwise we'd delete the required providers block after
+		// writing it.
+		for i, path := range rewritePaths {
+			if path == filename {
+				rewritePaths = append(rewritePaths[:i], rewritePaths[i+1:]...)
+				break
+			}
+		}
+
+		var out *hclwrite.File
+
+		// If the output file doesn't exist, just create a new empty file
+		if _, err := os.Stat(filename); os.IsNotExist(err) {
+			out = hclwrite.NewEmptyFile()
+		} else if err != nil {
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Unable to read configuration file",
+				fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+			))
+			c.showDiagnostics(diags)
+			return 1
+		} else {
+			// Configuration file already exists, so load and parse it
+			config, err := ioutil.ReadFile(filename)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to read configuration file",
+					fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+			var parseDiags hcl.Diagnostics
+			out, parseDiags = hclwrite.ParseConfig(config, filename, hcl.InitialPos)
+			diags = diags.Append(parseDiags)
+		}
+
+		if diags.HasErrors() {
+			c.showDiagnostics(diags)
+			return 1
+		}
+
+		// Find all required_providers blocks, and store them alongside a map
+		// back to the parent terraform block.
+		var requiredProviderBlocks []*hclwrite.Block
+		parentBlocks := make(map[*hclwrite.Block]*hclwrite.Block)
+		root := out.Body()
+		for _, rootBlock := range root.Blocks() {
+			if rootBlock.Type() != "terraform" {
+				continue
+			}
+			for _, childBlock := range rootBlock.Body().Blocks() {
+				if childBlock.Type() == "required_providers" {
+					requiredProviderBlocks = append(requiredProviderBlocks, childBlock)
+					parentBlocks[childBlock] = rootBlock
+				}
+			}
+		}
+
+		// First required provider block, and the rest found in this file.
+		var first *hclwrite.Block
+		var rest []*hclwrite.Block
+
+		if len(requiredProviderBlocks) > 0 {
+			// If we already have one or more required provider blocks, we'll rewrite
+			// the first one, and remove the rest.
+			first, rest = requiredProviderBlocks[0], requiredProviderBlocks[1:]
+		} else {
+			// Otherwise, find or a create a terraform block, and add a new
+			// empty required providers block to it.
+			var tfBlock *hclwrite.Block
+			for _, rootBlock := range root.Blocks() {
+				if rootBlock.Type() == "terraform" {
+					tfBlock = rootBlock
+					break
+				}
+			}
+			if tfBlock == nil {
+				tfBlock = root.AppendNewBlock("terraform", nil)
+			}
+			first = tfBlock.Body().AppendNewBlock("required_providers", nil)
+		}
+
+		// Find the body of the first block to prepare for rewriting it
+		body := first.Body()
+
+		// Build a sorted list of provider local names, for consistent ordering
+		var localNames []string
+		for localName := range requiredProviders {
+			localNames = append(localNames, localName)
+		}
+		sort.Strings(localNames)
+
+		// Populate the required providers block
+		for _, localName := range localNames {
+			requiredProvider := requiredProviders[localName]
+			var attributes = make(map[string]cty.Value)
+
+			if !requiredProvider.Type.IsZero() {
+				attributes["source"] = cty.StringVal(requiredProvider.Type.ForDisplay())
+			}
+
+			if version := requiredProvider.Requirement.Required.String(); version != "" {
+				attributes["version"] = cty.StringVal(version)
+			}
+
+			var attributesObject cty.Value
+			if len(attributes) > 0 {
+				attributesObject = cty.ObjectVal(attributes)
+			} else {
+				attributesObject = cty.EmptyObjectVal
+			}
+			body.SetAttributeValue(localName, attributesObject)
+
+			// If we don't have a source attribute, manually construct a commented
+			// block explaining what to do
+			if _, hasSource := attributes["source"]; !hasSource {
+				// Generate the token stream for the required provider
+				rp := body.GetAttribute(localName)
+				expr := rp.Expr().BuildTokens(nil)
+
+				// Partition the tokens into before and after the opening brace
+				before, after := partitionTokensAfter(expr, hclsyntax.TokenOBrace)
+
+				// If the value is an empty object, add a newline between the
+				// braces so that the comment is not on the same line as either
+				// brace.
+				if len(before) == 1 && len(after) == 1 {
+					newline := &hclwrite.Token{
+						Type:  hclsyntax.TokenNewline,
+						Bytes: []byte{'\n'},
+					}
+					after = append(hclwrite.Tokens{newline}, after...)
+				}
+
+				// Generate the comment and insert it at the start of the object
+				comment := noSourceDetectedComment(localName)
+				commentedBlock := append(before, comment...)
+				commentedBlock = append(commentedBlock, after...)
+
+				// Set the required provider object to this raw token stream
+				body.SetAttributeRaw(localName, commentedBlock)
+			}
+		}
+
+		// Remove the rest of the blocks (and the parent block, if it's empty)
+		for _, rpBlock := range rest {
+			tfBlock := parentBlocks[rpBlock]
+			tfBody := tfBlock.Body()
+			tfBody.RemoveBlock(rpBlock)
+
+			// If the terraform block has no blocks and no attributes, it's
+			// basically empty (aside from comments and whitespace), so it's
+			// more useful to remove it than leave it in.
+			if len(tfBody.Blocks()) == 0 && len(tfBody.Attributes()) == 0 {
+				root.RemoveBlock(tfBlock)
+			}
+		}
+
+		// Write the config back to the file
+		f, err := os.OpenFile(filename, os.O_TRUNC|os.O_CREATE|os.O_WRONLY, 0644)
+		if err != nil {
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Unable to open configuration file for writing",
+				fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+			))
+			c.showDiagnostics(diags)
+			return 1
+		}
+		_, err = out.WriteTo(f)
+		if err != nil {
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Error,
+				"Unable to rewrite configuration file",
+				fmt.Sprintf("Error when rewriting configuration file %q: %s", filename, err),
+			))
+			c.showDiagnostics(diags)
+			return 1
+		}
+
+		// After successfully writing the new configuration, remove all other
+		// required provider blocks from remaining configuration files.
+		for _, path := range rewritePaths {
+			// Read and parse the existing file
+			config, err := ioutil.ReadFile(path)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to read configuration file",
+					fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+			file, parseDiags := hclwrite.ParseConfig(config, filename, hcl.InitialPos)
+			diags = diags.Append(parseDiags)
+			if diags.HasErrors() {
+				c.showDiagnostics(diags)
+				return 1
+			}
+
+			// Find and remove all terraform.required_providers blocks
+			root := file.Body()
+			for _, rootBlock := range root.Blocks() {
+				if rootBlock.Type() != "terraform" {
+					continue
+				}
+				tfBody := rootBlock.Body()
+				for _, childBlock := range tfBody.Blocks() {
+					if childBlock.Type() == "required_providers" {
+						rootBlock.Body().RemoveBlock(childBlock)
+
+						// If the terraform block is now empty, remove it
+						if len(tfBody.Blocks()) == 0 && len(tfBody.Attributes()) == 0 {
+							root.RemoveBlock(rootBlock)
+						}
+					}
+				}
+			}
+
+			// Write the config back to the file
+			f, err := os.OpenFile(path, os.O_TRUNC|os.O_CREATE|os.O_WRONLY, 0644)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to open configuration file for writing",
+					fmt.Sprintf("Error when reading configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+			_, err = file.WriteTo(f)
+			if err != nil {
+				diags = diags.Append(tfdiags.Sourceless(
+					tfdiags.Error,
+					"Unable to rewrite configuration file",
+					fmt.Sprintf("Error when rewriting configuration file %q: %s", filename, err),
+				))
+				c.showDiagnostics(diags)
+				return 1
+			}
+		}
+	}
 
 	c.showDiagnostics(diags)
 	if diags.HasErrors() {
-		return 2
+		return 1
 	}
 
 	if len(diags) != 0 {
@@ -138,64 +500,78 @@ necessary adjustments, and then commit.
 	return 0
 }
 
-// For providers which need a source attribute, detect and return source
-// FIXME: currently does not filter or detect sources
-func detectProviderSources(providers moduledeps.Providers) (map[string]string, tfdiags.Diagnostics) {
-	sources := make(map[string]string)
-	for provider := range providers {
-		sources[provider.Type] = provider.String()
-	}
-	return sources, nil
-}
-
-var providersTemplate = template.Must(template.New("providers.tf").Parse(`terraform {
-  required_providers {
-    {{- range $type, $source := .}}
-    {{$type}} = {
-      source = "{{$source}}"
-    }
-    {{- end}}
-  }
-}
-`))
-
-// Generate a file with terraform.required_providers blocks for each provider
-func generateRequiredProviders(providerSources map[string]string, dir string) tfdiags.Diagnostics {
+// For providers which need a source attribute, detect the source
+func (c *ZeroThirteenUpgradeCommand) detectProviderSources(requiredProviders map[string]*configs.RequiredProvider) tfdiags.Diagnostics {
+	source := c.providerInstallSource()
 	var diags tfdiags.Diagnostics
 
-	// Find unused file named "providers.tf", or fall back to e.g. "providers-1.tf"
-	path := filepath.Join(dir, "providers.tf")
-	if _, err := os.Stat(path); !os.IsNotExist(err) {
-		for i := 1; ; i++ {
-			path = filepath.Join(dir, fmt.Sprintf("providers-%d.tf", i))
-			if _, err := os.Stat(path); os.IsNotExist(err) {
-				break
+	for name, rp := range requiredProviders {
+		// If there's already an explicit source, skip it
+		if rp.Source != "" {
+			continue
+		}
+
+		// Construct a legacy provider FQN using the required provider local
+		// name.  This ignores any auto-generated provider FQN from the load &
+		// parse process, because we know that without an explicit source it is
+		// not explicitly specified.
+		addr := addrs.NewLegacyProvider(name)
+		p, err := getproviders.LookupLegacyProvider(addr, source)
+		if err == nil {
+			rp.Type = p
+		} else {
+			if _, ok := err.(getproviders.ErrProviderNotKnown); ok {
+				// Setting the provider address to a zero value struct
+				// indicates that there is no known FQN for this provider,
+				// which will cause us to write an explanatory comment in the
+				// HCL output advising the user what to do about this.
+				rp.Type = addrs.Provider{}
 			}
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Warning,
+				"Could not detect provider source",
+				fmt.Sprintf("Error looking up provider source for %q: %s", name, err),
+			))
 		}
 	}
 
-	f, err := os.Create(path)
-	if err != nil {
-		diags = diags.Append(tfdiags.Sourceless(
-			tfdiags.Error,
-			"Unable to create providers file",
-			fmt.Sprintf("Error when generating providers configuration at '%s': %s", path, err),
-		))
-		return diags
-	}
-	defer f.Close()
+	return diags
+}
 
-	err = providersTemplate.Execute(f, providerSources)
-	if err != nil {
-		diags = diags.Append(tfdiags.Sourceless(
-			tfdiags.Error,
-			"Unable to create providers file",
-			fmt.Sprintf("Error when generating providers configuration at '%s': %s", path, err),
-		))
-		return diags
+// Take a list of tokens and a separator token, and return two lists: one up to
+// and including the first instance of the separator, and the rest of the
+// tokens. If the separator is not present, return the entire list in the first
+// return value.
+func partitionTokensAfter(tokens hclwrite.Tokens, separator hclsyntax.TokenType) (hclwrite.Tokens, hclwrite.Tokens) {
+	for i := 0; i < len(tokens); i++ {
+		if tokens[i].Type == separator {
+			return tokens[0 : i+1], tokens[i+1:]
+		}
 	}
 
-	return nil
+	return tokens, nil
+}
+
+// Generate a list of tokens for a comment explaining that a provider source
+// could not be detected.
+func noSourceDetectedComment(name string) hclwrite.Tokens {
+	comment := fmt.Sprintf(`# TF-UPGRADE-TODO
+#
+# No source detected for this provider. You must add a source address
+# in the following format:
+#
+# source = "your-registry.example.com/organization/%s"
+#
+# For more information, see the provider source documentation:
+#
+# https://www.terraform.io/docs/configuration/providers.html#provider-source`, name)
+
+	var tokens hclwrite.Tokens
+	for _, line := range strings.Split(comment, "\n") {
+		tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenNewline, Bytes: []byte{'\n'}})
+		tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenComment, Bytes: []byte(line)})
+	}
+	return tokens
 }
 
 func (c *ZeroThirteenUpgradeCommand) Help() string {
diff --git a/command/013_config_upgrade_test.go b/command/013_config_upgrade_test.go
index 219beb551..b302cfc4b 100644
--- a/command/013_config_upgrade_test.go
+++ b/command/013_config_upgrade_test.go
@@ -1,44 +1,115 @@
 package command
 
 import (
-	"bytes"
 	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
 	"os"
+	"path"
+	"path/filepath"
 	"strings"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
+	svchost "github.com/hashicorp/terraform-svchost"
+	"github.com/hashicorp/terraform-svchost/disco"
 	"github.com/hashicorp/terraform/helper/copy"
+	"github.com/hashicorp/terraform/internal/getproviders"
 	"github.com/mitchellh/cli"
 )
 
-func TestZeroThirteenUpgrade_success(t *testing.T) {
-	testCases := map[string]struct {
-		path string
-		args []string
-		out  string
-	}{
-		"implicit": {
-			path: "013upgrade-implicit-providers",
-			out:  "providers.tf",
-		},
-		"explicit": {
-			path: "013upgrade-explicit-providers",
-			out:  "providers.tf",
-		},
-		"subdir": {
-			path: "013upgrade-subdir",
-			args: []string{"subdir"},
-			out:  "subdir/providers.tf",
-		},
-		"fileExists": {
-			path: "013upgrade-file-exists",
-			out:  "providers-1.tf",
-		},
+// This map from provider type name to namespace is used by the fake registry
+// when called via LookupLegacyProvider. Providers not in this map will return
+// a 404 Not Found error.
+var legacyProviderNamespaces = map[string]string{
+	"foo": "hashicorp",
+	"bar": "hashicorp",
+	"baz": "terraform-providers",
+}
+
+func verifyExpectedFiles(t *testing.T, expectedPath string) {
+	// Compare output and expected file trees
+	var outputFiles, expectedFiles []string
+
+	// Gather list of output files in the current working directory
+	err := filepath.Walk(".", func(path string, info os.FileInfo, err error) error {
+		if !info.IsDir() {
+			outputFiles = append(outputFiles, path)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal("error listing output files:", err)
 	}
-	for name, tc := range testCases {
+
+	// Gather list of expected files
+	revertChdir := testChdir(t, expectedPath)
+	err = filepath.Walk(".", func(path string, info os.FileInfo, err error) error {
+		if !info.IsDir() {
+			expectedFiles = append(expectedFiles, path)
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal("error listing expected files:", err)
+	}
+	revertChdir()
+
+	// If the file trees don't match, give up early
+	if diff := cmp.Diff(expectedFiles, outputFiles); diff != "" {
+		t.Fatalf("expected and output file trees do not match\n%s", diff)
+	}
+
+	// Check that the contents of each file is correct
+	for _, filePath := range outputFiles {
+		output, err := ioutil.ReadFile(path.Join(".", filePath))
+		if err != nil {
+			t.Fatalf("failed to read output %s: %s", filePath, err)
+		}
+		expected, err := ioutil.ReadFile(path.Join(expectedPath, filePath))
+		if err != nil {
+			t.Fatalf("failed to read expected %s: %s", filePath, err)
+		}
+
+		if diff := cmp.Diff(expected, output); diff != "" {
+			t.Fatalf("expected and output file for %s do not match\n%s", filePath, diff)
+		}
+	}
+}
+
+func TestZeroThirteenUpgrade_success(t *testing.T) {
+	registrySource, close := testRegistrySource(t)
+	defer close()
+
+	testCases := map[string]string{
+		"implicit":              "013upgrade-implicit-providers",
+		"explicit":              "013upgrade-explicit-providers",
+		"provider not found":    "013upgrade-provider-not-found",
+		"implicit not found":    "013upgrade-implicit-not-found",
+		"file exists":           "013upgrade-file-exists",
+		"no providers":          "013upgrade-no-providers",
+		"submodule":             "013upgrade-submodule",
+		"providers with source": "013upgrade-providers-with-source",
+		"preserves comments":    "013upgrade-preserves-comments",
+		"multiple blocks":       "013upgrade-multiple-blocks",
+		"multiple files":        "013upgrade-multiple-files",
+		"existing providers.tf": "013upgrade-existing-providers-tf",
+		"skipped files":         "013upgrade-skipped-files",
+	}
+	for name, testPath := range testCases {
 		t.Run(name, func(t *testing.T) {
+			inputPath, err := filepath.Abs(testFixturePath(path.Join(testPath, "input")))
+			if err != nil {
+				t.Fatalf("failed to find input path %s: %s", testPath, err)
+			}
+
+			expectedPath, err := filepath.Abs(testFixturePath(path.Join(testPath, "expected")))
+			if err != nil {
+				t.Fatalf("failed to find expected path %s: %s", testPath, err)
+			}
+
 			td := tempDir(t)
-			copy.CopyDir(testFixturePath(tc.path), td)
+			copy.CopyDir(inputPath, td)
 			defer os.RemoveAll(td)
 			defer testChdir(t, td)()
 
@@ -46,11 +117,12 @@ func TestZeroThirteenUpgrade_success(t *testing.T) {
 			c := &ZeroThirteenUpgradeCommand{
 				Meta: Meta{
 					testingOverrides: metaOverridesForProvider(testProvider()),
+					ProviderSource:   registrySource,
 					Ui:               ui,
 				},
 			}
 
-			if code := c.Run(tc.args); code != 0 {
+			if code := c.Run(nil); code != 0 {
 				t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
 			}
 
@@ -59,22 +131,95 @@ func TestZeroThirteenUpgrade_success(t *testing.T) {
 				t.Fatal("unexpected output:", output)
 			}
 
-			actual, err := ioutil.ReadFile(tc.out)
-			if err != nil {
-				t.Fatalf("failed to read output %s: %s", tc.out, err)
-			}
-			expected, err := ioutil.ReadFile("expected/providers.tf")
-			if err != nil {
-				t.Fatal("failed to read expected/providers.tf", err)
-			}
-
-			if !bytes.Equal(actual, expected) {
-				t.Fatalf("actual output: \n%s\nexpected output: \n%s", string(actual), string(expected))
-			}
+			verifyExpectedFiles(t, expectedPath)
 		})
 	}
 }
 
+// Ensure that non-default upgrade paths are supported, and that the output is
+// in the correct place. This test is very similar to the table tests above,
+// but with a different expected output path, and with an argument passed to
+// the Run call.
+func TestZeroThirteenUpgrade_submodule(t *testing.T) {
+	registrySource, close := testRegistrySource(t)
+	defer close()
+
+	testPath := "013upgrade-submodule"
+
+	inputPath, err := filepath.Abs(testFixturePath(path.Join(testPath, "input")))
+	if err != nil {
+		t.Fatalf("failed to find input path %s: %s", testPath, err)
+	}
+
+	// The expected output for processing a submodule is different
+	expectedPath, err := filepath.Abs(testFixturePath(path.Join(testPath, "expected-module")))
+	if err != nil {
+		t.Fatalf("failed to find expected path %s: %s", testPath, err)
+	}
+
+	td := tempDir(t)
+	copy.CopyDir(inputPath, td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &ZeroThirteenUpgradeCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			ProviderSource:   registrySource,
+			Ui:               ui,
+		},
+	}
+
+	// Here we pass a target module directory to process
+	if code := c.Run([]string{"module"}); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	output := ui.OutputWriter.String()
+	if !strings.Contains(output, "Upgrade complete") {
+		t.Fatal("unexpected output:", output)
+	}
+
+	verifyExpectedFiles(t, expectedPath)
+}
+
+// Verify that JSON and override files are skipped with a warning. Generated
+// output for this config is verified in the table driven tests above.
+func TestZeroThirteenUpgrade_skippedFiles(t *testing.T) {
+	inputPath := testFixturePath(path.Join("013upgrade-skipped-files", "input"))
+
+	td := tempDir(t)
+	copy.CopyDir(inputPath, td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &ZeroThirteenUpgradeCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	if code := c.Run(nil); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	output := ui.OutputWriter.String()
+	if !strings.Contains(output, "Upgrade complete") {
+		t.Fatal("unexpected output:", output)
+	}
+
+	errMsg := ui.ErrorWriter.String()
+	if !strings.Contains(errMsg, `The JSON configuration file "variables.tf.json" was skipped`) {
+		t.Fatal("missing JSON skipped file warning:", errMsg)
+	}
+	if !strings.Contains(errMsg, `The override configuration file "bar_override.tf" was skipped`) {
+		t.Fatal("missing override skipped file warning:", errMsg)
+	}
+}
+
 func TestZeroThirteenUpgrade_invalidFlags(t *testing.T) {
 	td := tempDir(t)
 	os.MkdirAll(td, 0755)
@@ -147,54 +292,66 @@ func TestZeroThirteenUpgrade_empty(t *testing.T) {
 	}
 }
 
-func TestZeroThirteenUpgrade_invalidProviderVersion(t *testing.T) {
-	td := tempDir(t)
-	copy.CopyDir(testFixturePath("013upgrade-invalid"), td)
-	defer os.RemoveAll(td)
-	defer testChdir(t, td)()
+// testServices starts up a local HTTP server running a fake provider registry
+// service which responds only to discovery requests and legacy provider lookup
+// API calls.
+//
+// The final return value is a function to call at the end of a test function
+// to shut down the test server. After you call that function, the discovery
+// object becomes useless.
+func testServices(t *testing.T) (services *disco.Disco, cleanup func()) {
+	server := httptest.NewServer(http.HandlerFunc(fakeRegistryHandler))
 
-	ui := new(cli.MockUi)
-	c := &ZeroThirteenUpgradeCommand{
-		Meta: Meta{
-			testingOverrides: metaOverridesForProvider(testProvider()),
-			Ui:               ui,
-		},
-	}
+	services = disco.New()
+	services.ForceHostServices(svchost.Hostname("registry.terraform.io"), map[string]interface{}{
+		"providers.v1": server.URL + "/providers/v1/",
+	})
 
-	if code := c.Run(nil); code == 0 {
-		t.Fatal("expected error, got:", ui.OutputWriter)
-	}
-
-	errMsg := ui.ErrorWriter.String()
-	if !strings.Contains(errMsg, "Invalid provider version constraint") {
-		t.Fatal("unexpected error:", errMsg)
+	return services, func() {
+		server.Close()
 	}
 }
 
-func TestZeroThirteenUpgrade_noProviders(t *testing.T) {
-	td := tempDir(t)
-	copy.CopyDir(testFixturePath("013upgrade-no-providers"), td)
-	defer os.RemoveAll(td)
-	defer testChdir(t, td)()
+// testRegistrySource is a wrapper around testServices that uses the created
+// discovery object to produce a Source instance that is ready to use with the
+// fake registry services.
+//
+// As with testServices, the final return value is a function to call at the end
+// of your test in order to shut down the test server.
+func testRegistrySource(t *testing.T) (source *getproviders.RegistrySource, cleanup func()) {
+	services, close := testServices(t)
+	source = getproviders.NewRegistrySource(services)
+	return source, close
+}
 
-	ui := new(cli.MockUi)
-	c := &ZeroThirteenUpgradeCommand{
-		Meta: Meta{
-			testingOverrides: metaOverridesForProvider(testProvider()),
-			Ui:               ui,
-		},
+func fakeRegistryHandler(resp http.ResponseWriter, req *http.Request) {
+	path := req.URL.EscapedPath()
+
+	if !strings.HasPrefix(path, "/providers/v1/") {
+		resp.WriteHeader(404)
+		resp.Write([]byte(`not a provider registry endpoint`))
+		return
 	}
 
-	if code := c.Run(nil); code != 0 {
-		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	pathParts := strings.Split(path, "/")[3:]
+
+	if len(pathParts) != 2 {
+		resp.WriteHeader(404)
+		resp.Write([]byte(`unrecognized path scheme`))
+		return
 	}
 
-	output := ui.OutputWriter.String()
-	if !strings.Contains(output, "No non-default providers found") {
-		t.Fatal("unexpected output:", output)
+	if pathParts[0] != "-" {
+		resp.WriteHeader(404)
+		resp.Write([]byte(`this registry only supports legacy namespace lookup requests`))
 	}
 
-	if _, err := os.Stat("providers.tf"); !os.IsNotExist(err) {
-		t.Fatal("unexpected providers.tf created")
+	if namespace, ok := legacyProviderNamespaces[pathParts[1]]; ok {
+		resp.Header().Set("Content-Type", "application/json")
+		resp.WriteHeader(200)
+		resp.Write([]byte(`{"namespace":"` + namespace + `"}`))
+	} else {
+		resp.WriteHeader(404)
+		resp.Write([]byte(`provider not found`))
 	}
 }
diff --git a/command/testdata/013upgrade-existing-providers-tf/expected/main.tf b/command/testdata/013upgrade-existing-providers-tf/expected/main.tf
new file mode 100644
index 000000000..109e0699c
--- /dev/null
+++ b/command/testdata/013upgrade-existing-providers-tf/expected/main.tf
@@ -0,0 +1,5 @@
+resource foo_resource b {}
+resource bar_resource c {}
+resource bar_resource ab {
+  provider = baz
+}
diff --git a/command/testdata/013upgrade-existing-providers-tf/expected/providers.tf b/command/testdata/013upgrade-existing-providers-tf/expected/providers.tf
new file mode 100644
index 000000000..8394c756e
--- /dev/null
+++ b/command/testdata/013upgrade-existing-providers-tf/expected/providers.tf
@@ -0,0 +1,16 @@
+# This is a file called providers.tf which does not originally have a
+# required_providers block. 
+resource foo_resource a {}
+terraform {
+  required_providers {
+    bar = {
+      source = "hashicorp/bar"
+    }
+    baz = {
+      source = "terraform-providers/baz"
+    }
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-existing-providers-tf/input/main.tf b/command/testdata/013upgrade-existing-providers-tf/input/main.tf
new file mode 100644
index 000000000..109e0699c
--- /dev/null
+++ b/command/testdata/013upgrade-existing-providers-tf/input/main.tf
@@ -0,0 +1,5 @@
+resource foo_resource b {}
+resource bar_resource c {}
+resource bar_resource ab {
+  provider = baz
+}
diff --git a/command/testdata/013upgrade-existing-providers-tf/input/providers.tf b/command/testdata/013upgrade-existing-providers-tf/input/providers.tf
new file mode 100644
index 000000000..04203320a
--- /dev/null
+++ b/command/testdata/013upgrade-existing-providers-tf/input/providers.tf
@@ -0,0 +1,3 @@
+# This is a file called providers.tf which does not originally have a
+# required_providers block. 
+resource foo_resource a {}
diff --git a/command/testdata/013upgrade-explicit-providers/expected/main.tf b/command/testdata/013upgrade-explicit-providers/expected/main.tf
new file mode 100644
index 000000000..83d1158f4
--- /dev/null
+++ b/command/testdata/013upgrade-explicit-providers/expected/main.tf
@@ -0,0 +1,19 @@
+provider foo {
+  version = "1.2.3"
+}
+
+terraform {
+  required_providers {
+    bar = {
+      source  = "hashicorp/bar"
+      version = "1.0.0"
+    }
+    baz = {
+      source  = "terraform-providers/baz"
+      version = "~> 2.0.0"
+    }
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-explicit-providers/expected/providers.tf b/command/testdata/013upgrade-explicit-providers/expected/providers.tf
deleted file mode 100644
index c12b7df0a..000000000
--- a/command/testdata/013upgrade-explicit-providers/expected/providers.tf
+++ /dev/null
@@ -1,13 +0,0 @@
-terraform {
-  required_providers {
-    bar = {
-      source = "registry.terraform.io/-/bar"
-    }
-    baz = {
-      source = "registry.terraform.io/-/baz"
-    }
-    foo = {
-      source = "registry.terraform.io/-/foo"
-    }
-  }
-}
diff --git a/command/testdata/013upgrade-explicit-providers/input/main.tf b/command/testdata/013upgrade-explicit-providers/input/main.tf
new file mode 100644
index 000000000..f3c982a04
--- /dev/null
+++ b/command/testdata/013upgrade-explicit-providers/input/main.tf
@@ -0,0 +1,12 @@
+provider foo {
+  version = "1.2.3"
+}
+
+terraform {
+  required_providers {
+    bar = "1.0.0"
+    baz = {
+      version = "~> 2.0.0"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-file-exists/expected/providers.tf b/command/testdata/013upgrade-file-exists/expected/providers.tf
index 5c4b69105..f0f674de4 100644
--- a/command/testdata/013upgrade-file-exists/expected/providers.tf
+++ b/command/testdata/013upgrade-file-exists/expected/providers.tf
@@ -1,10 +1,12 @@
+provider foo {}
+provider bar {}
 terraform {
   required_providers {
-    alpha = {
-      source = "registry.terraform.io/-/alpha"
+    bar = {
+      source = "hashicorp/bar"
     }
-    beta = {
-      source = "registry.terraform.io/-/beta"
+    foo = {
+      source = "hashicorp/foo"
     }
   }
 }
diff --git a/command/testdata/013upgrade-file-exists/input/providers.tf b/command/testdata/013upgrade-file-exists/input/providers.tf
new file mode 100644
index 000000000..e9f22a979
--- /dev/null
+++ b/command/testdata/013upgrade-file-exists/input/providers.tf
@@ -0,0 +1,2 @@
+provider foo {}
+provider bar {}
diff --git a/command/testdata/013upgrade-file-exists/providers.tf b/command/testdata/013upgrade-file-exists/providers.tf
deleted file mode 100644
index a13ab6d45..000000000
--- a/command/testdata/013upgrade-file-exists/providers.tf
+++ /dev/null
@@ -1,2 +0,0 @@
-provider alpha {}
-provider beta {}
diff --git a/command/testdata/013upgrade-implicit-not-found/expected/main.tf b/command/testdata/013upgrade-implicit-not-found/expected/main.tf
new file mode 100644
index 000000000..afb4083b5
--- /dev/null
+++ b/command/testdata/013upgrade-implicit-not-found/expected/main.tf
@@ -0,0 +1 @@
+resource something_resource a {}
diff --git a/command/testdata/013upgrade-implicit-not-found/expected/providers.tf b/command/testdata/013upgrade-implicit-not-found/expected/providers.tf
new file mode 100644
index 000000000..d076393f4
--- /dev/null
+++ b/command/testdata/013upgrade-implicit-not-found/expected/providers.tf
@@ -0,0 +1,16 @@
+terraform {
+  required_providers {
+    something = {
+      # TF-UPGRADE-TODO
+      #
+      # No source detected for this provider. You must add a source address
+      # in the following format:
+      #
+      # source = "your-registry.example.com/organization/something"
+      #
+      # For more information, see the provider source documentation:
+      #
+      # https://www.terraform.io/docs/configuration/providers.html#provider-source
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-implicit-not-found/input/main.tf b/command/testdata/013upgrade-implicit-not-found/input/main.tf
new file mode 100644
index 000000000..afb4083b5
--- /dev/null
+++ b/command/testdata/013upgrade-implicit-not-found/input/main.tf
@@ -0,0 +1 @@
+resource something_resource a {}
diff --git a/command/testdata/013upgrade-implicit-providers/expected/main.tf b/command/testdata/013upgrade-implicit-providers/expected/main.tf
new file mode 100644
index 000000000..109e0699c
--- /dev/null
+++ b/command/testdata/013upgrade-implicit-providers/expected/main.tf
@@ -0,0 +1,5 @@
+resource foo_resource b {}
+resource bar_resource c {}
+resource bar_resource ab {
+  provider = baz
+}
diff --git a/command/testdata/013upgrade-implicit-providers/expected/providers.tf b/command/testdata/013upgrade-implicit-providers/expected/providers.tf
index 1f1064007..9b2a488e5 100644
--- a/command/testdata/013upgrade-implicit-providers/expected/providers.tf
+++ b/command/testdata/013upgrade-implicit-providers/expected/providers.tf
@@ -1,10 +1,13 @@
 terraform {
   required_providers {
-    cloud = {
-      source = "registry.terraform.io/-/cloud"
+    bar = {
+      source = "hashicorp/bar"
     }
-    some = {
-      source = "registry.terraform.io/-/some"
+    baz = {
+      source = "terraform-providers/baz"
+    }
+    foo = {
+      source = "hashicorp/foo"
     }
   }
 }
diff --git a/command/testdata/013upgrade-implicit-providers/input/main.tf b/command/testdata/013upgrade-implicit-providers/input/main.tf
new file mode 100644
index 000000000..109e0699c
--- /dev/null
+++ b/command/testdata/013upgrade-implicit-providers/input/main.tf
@@ -0,0 +1,5 @@
+resource foo_resource b {}
+resource bar_resource c {}
+resource bar_resource ab {
+  provider = baz
+}
diff --git a/command/testdata/013upgrade-implicit-providers/main.tf b/command/testdata/013upgrade-implicit-providers/main.tf
deleted file mode 100644
index be9fa73e1..000000000
--- a/command/testdata/013upgrade-implicit-providers/main.tf
+++ /dev/null
@@ -1,2 +0,0 @@
-resource some_resource a {}
-resource cloud_horse x {}
diff --git a/command/testdata/013upgrade-invalid/main.tf b/command/testdata/013upgrade-invalid/main.tf
deleted file mode 100644
index b81807c3e..000000000
--- a/command/testdata/013upgrade-invalid/main.tf
+++ /dev/null
@@ -1,3 +0,0 @@
-provider "invalid" {
-	version = "invalid"
-}
diff --git a/command/testdata/013upgrade-multiple-blocks/expected/bar-baz.tf b/command/testdata/013upgrade-multiple-blocks/expected/bar-baz.tf
new file mode 100644
index 000000000..6887569dd
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-blocks/expected/bar-baz.tf
@@ -0,0 +1,5 @@
+resource bar_instance a {}
+resource baz_instance b {}
+terraform {
+  required_version = "> 0.12.0"
+}
diff --git a/command/testdata/013upgrade-multiple-blocks/expected/foo.tf b/command/testdata/013upgrade-multiple-blocks/expected/foo.tf
new file mode 100644
index 000000000..5d9203723
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-blocks/expected/foo.tf
@@ -0,0 +1 @@
+resource foo_instance c {}
diff --git a/command/testdata/013upgrade-multiple-blocks/expected/providers.tf b/command/testdata/013upgrade-multiple-blocks/expected/providers.tf
new file mode 100644
index 000000000..195b21389
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-blocks/expected/providers.tf
@@ -0,0 +1,15 @@
+terraform {
+  required_providers {
+    bar = {
+      source = "registry.acme.corp/acme/bar"
+    }
+    baz = {
+      source  = "terraform-providers/baz"
+      version = "~> 2.0.0"
+    }
+    foo = {
+      source  = "hashicorp/foo"
+      version = "0.5"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-multiple-blocks/input/bar-baz.tf b/command/testdata/013upgrade-multiple-blocks/input/bar-baz.tf
new file mode 100644
index 000000000..d0e6fe8c6
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-blocks/input/bar-baz.tf
@@ -0,0 +1,10 @@
+resource bar_instance a {}
+resource baz_instance b {}
+terraform {
+  required_version = "> 0.12.0"
+  required_providers {
+    bar = {
+      source = "registry.acme.corp/acme/bar"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-multiple-blocks/input/foo.tf b/command/testdata/013upgrade-multiple-blocks/input/foo.tf
new file mode 100644
index 000000000..99aaf52a0
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-blocks/input/foo.tf
@@ -0,0 +1,11 @@
+terraform {
+  required_providers {
+    baz = "~> 2.0.0"
+  }
+}
+terraform {
+  required_providers {
+    foo = "0.5"
+  }
+}
+resource foo_instance c {}
diff --git a/command/testdata/013upgrade-multiple-files/expected/main.tf b/command/testdata/013upgrade-multiple-files/expected/main.tf
new file mode 100644
index 000000000..fa25239e9
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-files/expected/main.tf
@@ -0,0 +1,4 @@
+# This file starts with a resource and a required providers block, and should
+# end up with just the resource.
+resource foo_instance a {}
+
diff --git a/command/testdata/013upgrade-multiple-files/expected/providers.tf b/command/testdata/013upgrade-multiple-files/expected/providers.tf
new file mode 100644
index 000000000..b46e7180d
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-files/expected/providers.tf
@@ -0,0 +1,16 @@
+# This file starts with a resource and a required providers block, and should
+# end up with the full required providers configuration. This file is chosen
+# to keep the required providers block because its file name is "providers.tf".
+resource bar_instance b {}
+
+terraform {
+  required_providers {
+    bar = {
+      source = "registry.acme.corp/acme/bar"
+    }
+    foo = {
+      source  = "hashicorp/foo"
+      version = "1.0.0"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-multiple-files/input/main.tf b/command/testdata/013upgrade-multiple-files/input/main.tf
new file mode 100644
index 000000000..168ebff7e
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-files/input/main.tf
@@ -0,0 +1,9 @@
+# This file starts with a resource and a required providers block, and should
+# end up with just the resource.
+resource foo_instance a {}
+
+terraform {
+  required_providers {
+    foo = "1.0.0"
+  }
+}
diff --git a/command/testdata/013upgrade-multiple-files/input/providers.tf b/command/testdata/013upgrade-multiple-files/input/providers.tf
new file mode 100644
index 000000000..8823c6fe6
--- /dev/null
+++ b/command/testdata/013upgrade-multiple-files/input/providers.tf
@@ -0,0 +1,12 @@
+# This file starts with a resource and a required providers block, and should
+# end up with the full required providers configuration. This file is chosen
+# to keep the required providers block because its file name is "providers.tf".
+resource bar_instance b {}
+
+terraform {
+  required_providers {
+    bar = {
+      source = "registry.acme.corp/acme/bar"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-no-providers/main.tf b/command/testdata/013upgrade-no-providers/expected/main.tf
similarity index 100%
rename from command/testdata/013upgrade-no-providers/main.tf
rename to command/testdata/013upgrade-no-providers/expected/main.tf
diff --git a/command/testdata/013upgrade-no-providers/input/main.tf b/command/testdata/013upgrade-no-providers/input/main.tf
new file mode 100644
index 000000000..d7ad43496
--- /dev/null
+++ b/command/testdata/013upgrade-no-providers/input/main.tf
@@ -0,0 +1,11 @@
+variable "x" {
+  default = 3
+}
+
+variable "y" {
+  default = 5
+}
+
+output "product" {
+  value = var.x * var.y
+}
diff --git a/command/testdata/013upgrade-preserves-comments/expected/main.tf b/command/testdata/013upgrade-preserves-comments/expected/main.tf
new file mode 100644
index 000000000..13bbe0b95
--- /dev/null
+++ b/command/testdata/013upgrade-preserves-comments/expected/main.tf
@@ -0,0 +1,16 @@
+resource foo_instance a {}
+resource bar_instance b {}
+
+terraform {
+  # Provider requirements go here
+  required_providers {
+    # Pin bar to this version
+    bar = {
+      source  = "hashicorp/bar"
+      version = "0.5.0"
+    }
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-preserves-comments/input/main.tf b/command/testdata/013upgrade-preserves-comments/input/main.tf
new file mode 100644
index 000000000..c384d2819
--- /dev/null
+++ b/command/testdata/013upgrade-preserves-comments/input/main.tf
@@ -0,0 +1,10 @@
+resource foo_instance a {}
+resource bar_instance b {}
+
+terraform {
+  # Provider requirements go here
+  required_providers {
+    # Pin bar to this version
+    bar = "0.5.0"
+  }
+}
diff --git a/command/testdata/013upgrade-provider-not-found/expected/main.tf b/command/testdata/013upgrade-provider-not-found/expected/main.tf
new file mode 100644
index 000000000..951db47d7
--- /dev/null
+++ b/command/testdata/013upgrade-provider-not-found/expected/main.tf
@@ -0,0 +1,26 @@
+provider foo {}
+
+terraform {
+  required_providers {
+    bar = {
+      source  = "hashicorp/bar"
+      version = "1.0.0"
+    }
+    unknown = {
+      # TF-UPGRADE-TODO
+      #
+      # No source detected for this provider. You must add a source address
+      # in the following format:
+      #
+      # source = "your-registry.example.com/organization/unknown"
+      #
+      # For more information, see the provider source documentation:
+      #
+      # https://www.terraform.io/docs/configuration/providers.html#provider-source
+      version = "~> 2.0.0"
+    }
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-explicit-providers/main.tf b/command/testdata/013upgrade-provider-not-found/input/main.tf
similarity index 87%
rename from command/testdata/013upgrade-explicit-providers/main.tf
rename to command/testdata/013upgrade-provider-not-found/input/main.tf
index 80ee3b7fe..9b6e38104 100644
--- a/command/testdata/013upgrade-explicit-providers/main.tf
+++ b/command/testdata/013upgrade-provider-not-found/input/main.tf
@@ -3,7 +3,7 @@ provider foo {}
 terraform {
   required_providers {
     bar = "1.0.0"
-    baz = {
+    unknown = {
       version = "~> 2.0.0"
     }
   }
diff --git a/command/testdata/013upgrade-providers-with-source/expected/main.tf b/command/testdata/013upgrade-providers-with-source/expected/main.tf
new file mode 100644
index 000000000..3d366ebfa
--- /dev/null
+++ b/command/testdata/013upgrade-providers-with-source/expected/main.tf
@@ -0,0 +1,7 @@
+terraform {
+  required_providers {
+    aws = {
+      source = "registry.acme.corp/acme/aws"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-providers-with-source/input/main.tf b/command/testdata/013upgrade-providers-with-source/input/main.tf
new file mode 100644
index 000000000..3d366ebfa
--- /dev/null
+++ b/command/testdata/013upgrade-providers-with-source/input/main.tf
@@ -0,0 +1,7 @@
+terraform {
+  required_providers {
+    aws = {
+      source = "registry.acme.corp/acme/aws"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-skipped-files/expected/bar_override.tf b/command/testdata/013upgrade-skipped-files/expected/bar_override.tf
new file mode 100644
index 000000000..524ef2c57
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/expected/bar_override.tf
@@ -0,0 +1,3 @@
+provider bar {
+  version = "1.0.0"
+}
diff --git a/command/testdata/013upgrade-skipped-files/expected/main.tf b/command/testdata/013upgrade-skipped-files/expected/main.tf
new file mode 100644
index 000000000..83d1158f4
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/expected/main.tf
@@ -0,0 +1,19 @@
+provider foo {
+  version = "1.2.3"
+}
+
+terraform {
+  required_providers {
+    bar = {
+      source  = "hashicorp/bar"
+      version = "1.0.0"
+    }
+    baz = {
+      source  = "terraform-providers/baz"
+      version = "~> 2.0.0"
+    }
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-skipped-files/expected/variables.tf.json b/command/testdata/013upgrade-skipped-files/expected/variables.tf.json
new file mode 100644
index 000000000..6126f1f0c
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/expected/variables.tf.json
@@ -0,0 +1,12 @@
+{
+  "variable": {
+    "example": {
+      "default": "hello"
+    }
+  }
+  "terraform": {
+    "required_providers": {
+      "aws": "2.50.0"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-skipped-files/input/bar_override.tf b/command/testdata/013upgrade-skipped-files/input/bar_override.tf
new file mode 100644
index 000000000..524ef2c57
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/input/bar_override.tf
@@ -0,0 +1,3 @@
+provider bar {
+  version = "1.0.0"
+}
diff --git a/command/testdata/013upgrade-skipped-files/input/main.tf b/command/testdata/013upgrade-skipped-files/input/main.tf
new file mode 100644
index 000000000..f3c982a04
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/input/main.tf
@@ -0,0 +1,12 @@
+provider foo {
+  version = "1.2.3"
+}
+
+terraform {
+  required_providers {
+    bar = "1.0.0"
+    baz = {
+      version = "~> 2.0.0"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-skipped-files/input/variables.tf.json b/command/testdata/013upgrade-skipped-files/input/variables.tf.json
new file mode 100644
index 000000000..6126f1f0c
--- /dev/null
+++ b/command/testdata/013upgrade-skipped-files/input/variables.tf.json
@@ -0,0 +1,12 @@
+{
+  "variable": {
+    "example": {
+      "default": "hello"
+    }
+  }
+  "terraform": {
+    "required_providers": {
+      "aws": "2.50.0"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-subdir/expected/providers.tf b/command/testdata/013upgrade-subdir/expected/providers.tf
deleted file mode 100644
index 5c4b69105..000000000
--- a/command/testdata/013upgrade-subdir/expected/providers.tf
+++ /dev/null
@@ -1,10 +0,0 @@
-terraform {
-  required_providers {
-    alpha = {
-      source = "registry.terraform.io/-/alpha"
-    }
-    beta = {
-      source = "registry.terraform.io/-/beta"
-    }
-  }
-}
diff --git a/command/testdata/013upgrade-subdir/subdir/main.tf b/command/testdata/013upgrade-subdir/subdir/main.tf
deleted file mode 100644
index 19cf29fdc..000000000
--- a/command/testdata/013upgrade-subdir/subdir/main.tf
+++ /dev/null
@@ -1,2 +0,0 @@
-resource beta_resource b {}
-resource alpha_resource a {}
diff --git a/command/testdata/013upgrade-submodule/expected-module/main.tf b/command/testdata/013upgrade-submodule/expected-module/main.tf
new file mode 100644
index 000000000..6c4de24ad
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/expected-module/main.tf
@@ -0,0 +1,8 @@
+terraform {
+  required_providers {
+    bar = {
+      version = "~> 2.0.0"
+    }
+  }
+}
+
diff --git a/command/testdata/013upgrade-submodule/expected-module/module/main.tf b/command/testdata/013upgrade-submodule/expected-module/module/main.tf
new file mode 100644
index 000000000..0f5566e56
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/expected-module/module/main.tf
@@ -0,0 +1 @@
+resource foo_resource b {}
diff --git a/command/testdata/013upgrade-submodule/expected-module/module/providers.tf b/command/testdata/013upgrade-submodule/expected-module/module/providers.tf
new file mode 100644
index 000000000..3af569d04
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/expected-module/module/providers.tf
@@ -0,0 +1,7 @@
+terraform {
+  required_providers {
+    foo = {
+      source = "hashicorp/foo"
+    }
+  }
+}
diff --git a/command/testdata/013upgrade-submodule/expected/main.tf b/command/testdata/013upgrade-submodule/expected/main.tf
new file mode 100644
index 000000000..89b6f4027
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/expected/main.tf
@@ -0,0 +1,9 @@
+terraform {
+  required_providers {
+    bar = {
+      source  = "hashicorp/bar"
+      version = "~> 2.0.0"
+    }
+  }
+}
+
diff --git a/command/testdata/013upgrade-submodule/expected/module/main.tf b/command/testdata/013upgrade-submodule/expected/module/main.tf
new file mode 100644
index 000000000..0f5566e56
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/expected/module/main.tf
@@ -0,0 +1 @@
+resource foo_resource b {}
diff --git a/command/testdata/013upgrade-submodule/input/main.tf b/command/testdata/013upgrade-submodule/input/main.tf
new file mode 100644
index 000000000..6c4de24ad
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/input/main.tf
@@ -0,0 +1,8 @@
+terraform {
+  required_providers {
+    bar = {
+      version = "~> 2.0.0"
+    }
+  }
+}
+
diff --git a/command/testdata/013upgrade-submodule/input/module/main.tf b/command/testdata/013upgrade-submodule/input/module/main.tf
new file mode 100644
index 000000000..0f5566e56
--- /dev/null
+++ b/command/testdata/013upgrade-submodule/input/module/main.tf
@@ -0,0 +1 @@
+resource foo_resource b {}
diff --git a/configs/provider_requirements.go b/configs/provider_requirements.go
index d4218a6fc..531ba4f7f 100644
--- a/configs/provider_requirements.go
+++ b/configs/provider_requirements.go
@@ -12,6 +12,7 @@ import (
 // parent.
 type RequiredProvider struct {
 	Name        string
+	Source      string
 	Type        addrs.Provider
 	Requirement VersionConstraint
 	DeclRange   hcl.Range
@@ -67,7 +68,8 @@ func decodeRequiredProvidersBlock(block *hcl.Block) (*RequiredProviders, hcl.Dia
 				}
 			}
 			if expr.Type().HasAttribute("source") {
-				fqn, sourceDiags := addrs.ParseProviderSourceString(expr.GetAttr("source").AsString())
+				rp.Source = expr.GetAttr("source").AsString()
+				fqn, sourceDiags := addrs.ParseProviderSourceString(rp.Source)
 
 				if sourceDiags.HasErrors() {
 					hclDiags := sourceDiags.ToHCL()
diff --git a/configs/provider_requirements_test.go b/configs/provider_requirements_test.go
index a04714f55..d204223b9 100644
--- a/configs/provider_requirements_test.go
+++ b/configs/provider_requirements_test.go
@@ -21,6 +21,9 @@ var (
 		if x.Type != y.Type {
 			return false
 		}
+		if x.Source != y.Source {
+			return false
+		}
 		if x.Requirement.Required.String() != y.Requirement.Required.String() {
 			return false
 		}
@@ -90,6 +93,7 @@ func TestDecodeRequiredProvidersBlock(t *testing.T) {
 				RequiredProviders: map[string]*RequiredProvider{
 					"my_test": {
 						Name:        "my_test",
+						Source:      "mycloud/test",
 						Type:        addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
 						Requirement: testVC("2.0.0"),
 						DeclRange:   mockRange,
@@ -128,6 +132,7 @@ func TestDecodeRequiredProvidersBlock(t *testing.T) {
 					},
 					"my_test": {
 						Name:        "my_test",
+						Source:      "mycloud/test",
 						Type:        addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
 						Requirement: testVC("2.0.0"),
 						DeclRange:   mockRange,
@@ -183,6 +188,7 @@ func TestDecodeRequiredProvidersBlock(t *testing.T) {
 				RequiredProviders: map[string]*RequiredProvider{
 					"my_test": {
 						Name:        "my_test",
+						Source:      "some/invalid/provider/source/test",
 						Type:        addrs.Provider{},
 						Requirement: testVC("~>2.0.0"),
 						DeclRange:   mockRange,
@@ -240,6 +246,7 @@ func TestDecodeRequiredProvidersBlock(t *testing.T) {
 				RequiredProviders: map[string]*RequiredProvider{
 					"my_test": {
 						Name:      "my_test",
+						Source:    "mycloud/test",
 						Type:      addrs.NewProvider(addrs.DefaultRegistryHost, "mycloud", "test"),
 						DeclRange: mockRange,
 					},
diff --git a/internal/getproviders/legacy_lookup.go b/internal/getproviders/legacy_lookup.go
index 96901abab..71d9b15cd 100644
--- a/internal/getproviders/legacy_lookup.go
+++ b/internal/getproviders/legacy_lookup.go
@@ -80,6 +80,11 @@ func findLegacyProviderLookupSource(host svchost.Hostname, source Source) *Regis
 		// just use it.
 		return source
 
+	case *MemoizeSource:
+		// Also easy: the source is a memoize wrapper, so defer to its
+		// underlying source.
+		return findLegacyProviderLookupSource(host, source.underlying)
+
 	case MultiSource:
 		// Trickier case: if it's a multisource then we need to scan over
 		// its selectors until we find one that is a *RegistrySource _and_