config: Merge respects Terraform blocks, provider aliases, and more

Fixes #10715

`config.Merge` was not updated to support a number of new features. This
updates the codepath to merge various fields, including the `terraform`
block which was the issue in #10715.

The `Merge` API is called when an `_override` file is present to _merge_
configurations. Normally configurations are _appended_. Only an override
file triggers a _merge_.

I started working on a generic library to do this automatically awhile
back but never finished it. This might motivate me to do so. In the
interest of getting a fix out though, we'll continue the manual
approach.

config/config.go

@@ -915,7 +915,10 @@ func (o *Output) mergerMerge(m merger) merger {
 
 	result := *o
 	result.Name = o2.Name
+	result.Description = o2.Description
 	result.RawConfig = result.RawConfig.merge(o2.RawConfig)
+	result.Sensitive = o2.Sensitive
+	result.DependsOn = o2.DependsOn
 
 	return &result
 }
@@ -943,6 +946,10 @@ func (c *ProviderConfig) mergerMerge(m merger) merger {
 	result.Name = c2.Name
 	result.RawConfig = result.RawConfig.merge(c2.RawConfig)
 
+	if c2.Alias != "" {
+		result.Alias = c2.Alias
+	}
+
 	return &result
 }
 
@@ -978,6 +985,9 @@ func (v *Variable) Merge(v2 *Variable) *Variable {
 	// The names should be the same, but the second name always wins.
 	result.Name = v2.Name
 
+	if v2.DeclaredType != "" {
+		result.DeclaredType = v2.DeclaredType
+	}
 	if v2.Default != nil {
 		result.Default = v2.Default
 	}

config/merge.go

@@ -32,6 +32,12 @@ func Merge(c1, c2 *Config) (*Config, error) {
 		c.Atlas = c2.Atlas
 	}
 
+	// Merge the Terraform configuration, which is a complete overwrite.
+	c.Terraform = c1.Terraform
+	if c2.Terraform != nil {
+		c.Terraform = c2.Terraform
+	}
+
 	// NOTE: Everything below is pretty gross. Due to the lack of generics
 	// in Go, there is some hoop-jumping involved to make this merging a
 	// little more test-friendly and less repetitive. Ironically, making it

config/merge_test.go

@@ -153,6 +153,287 @@ func TestMerge(t *testing.T) {
 
 			false,
 		},
+
+		// Terraform block
+		{
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+				},
+			},
+			&Config{},
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+				},
+			},
+			&Config{
+				Terraform: &Terraform{
+					RequiredVersion: "A",
+				},
+			},
+			false,
+		},
+
+		// Provider alias
+		{
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			&Config{},
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "bar"},
+				},
+			},
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			&Config{
+				ProviderConfigs: []*ProviderConfig{
+					&ProviderConfig{Alias: "foo"},
+				},
+			},
+			false,
+		},
+
+		// Variable type
+		{
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			&Config{},
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "bar"},
+				},
+			},
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			&Config{
+				Variables: []*Variable{
+					&Variable{DeclaredType: "foo"},
+				},
+			},
+			false,
+		},
+
+		// Output description
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "bar"},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Description: "foo"},
+				},
+			},
+			false,
+		},
+
+		// Output depends_on
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"bar"}},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{DependsOn: []string{"foo"}},
+				},
+			},
+			false,
+		},
+
+		// Output sensitive
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{},
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			false,
+		},
+
+		{
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: false},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			&Config{
+				Outputs: []*Output{
+					&Output{Sensitive: true},
+				},
+			},
+			false,
+		},
 	}
 
 	for i, tc := range cases {

config/raw_config.go

@@ -240,14 +240,24 @@ func (r *RawConfig) interpolate(fn interpolationWalkerFunc) error {
 }
 
 func (r *RawConfig) merge(r2 *RawConfig) *RawConfig {
+	if r == nil && r2 == nil {
+		return nil
+	}
+
+	if r == nil {
+		r = &RawConfig{}
+	}
+
 	rawRaw, err := copystructure.Copy(r.Raw)
 	if err != nil {
 		panic(err)
 	}
 
 	raw := rawRaw.(map[string]interface{})
-	for k, v := range r2.Raw {
-		raw[k] = v
+	if r2 != nil {
+		for k, v := range r2.Raw {
+			raw[k] = v
+		}
 	}
 
 	result, err := NewRawConfig(raw)