helper/schema: Better mimic some undocumented behaviors in Core schema

Since the SDK's schema system conflates attributes and nested blocks, it's
possible to state some nonsensical schema situations such as:

- A nested block is both optional but has MinItems > 0
- A nested block is entirely computed but has MinItems or MaxItems set

Both of these weird situations are handled here in the same way that the
existing helper/schema validation code would've handled them: by
effectively disabling the MinItems/MaxItems checks where they would've
been ignored before.

the MinItems/MaxItems

helper/schema/core_schema.go

@@ -125,6 +125,20 @@ func (s *Schema) coreConfigSchemaBlock() *configschema.NestedBlock {
 		// blocks, but we can fake it by requiring at least one item.
 		ret.MinItems = 1
 	}
+	if s.Optional && s.MinItems > 0 {
+		// Historically helper/schema would ignore MinItems if Optional were
+		// set, so we must mimic this behavior here to ensure that providers
+		// relying on that undocumented behavior can continue to operate as
+		// they did before.
+		ret.MinItems = 0
+	}
+	if s.Computed && !s.Optional {
+		// MinItems/MaxItems are meaningless for computed nested blocks, since
+		// they are never set by the user anyway. This ensures that we'll never
+		// generate weird errors about them.
+		ret.MinItems = 0
+		ret.MaxItems = 0
+	}
 
 	return ret
 }

helper/schema/core_schema_test.go

@@ -224,6 +224,90 @@ func TestSchemaMapCoreConfigSchema(t *testing.T) {
 				},
 			}),
 		},
+		"sub-resource collections minitems+optional": {
+			// This particular case is an odd one where the provider gives
+			// conflicting information about whether a sub-resource is required,
+			// by marking it as optional but also requiring one item.
+			// Historically the optional-ness "won" here, and so we must
+			// honor that for compatibility with providers that relied on this
+			// undocumented interaction.
+			map[string]*Schema{
+				"list": {
+					Type:     TypeList,
+					Optional: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{},
+					},
+					MinItems: 1,
+					MaxItems: 1,
+				},
+				"set": {
+					Type:     TypeSet,
+					Optional: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{},
+					},
+					MinItems: 1,
+					MaxItems: 1,
+				},
+			},
+			testResource(&configschema.Block{
+				Attributes: map[string]*configschema.Attribute{},
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"list": {
+						Nesting:  configschema.NestingList,
+						Block:    configschema.Block{},
+						MinItems: 0,
+						MaxItems: 1,
+					},
+					"set": {
+						Nesting:  configschema.NestingSet,
+						Block:    configschema.Block{},
+						MinItems: 0,
+						MaxItems: 1,
+					},
+				},
+			}),
+		},
+		"sub-resource collections minitems+computed": {
+			map[string]*Schema{
+				"list": {
+					Type:     TypeList,
+					Computed: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{},
+					},
+					MinItems: 1,
+					MaxItems: 1,
+				},
+				"set": {
+					Type:     TypeSet,
+					Computed: true,
+					Elem: &Resource{
+						Schema: map[string]*Schema{},
+					},
+					MinItems: 1,
+					MaxItems: 1,
+				},
+			},
+			testResource(&configschema.Block{
+				Attributes: map[string]*configschema.Attribute{},
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"list": {
+						Nesting:  configschema.NestingList,
+						Block:    configschema.Block{},
+						MinItems: 0,
+						MaxItems: 0,
+					},
+					"set": {
+						Nesting:  configschema.NestingSet,
+						Block:    configschema.Block{},
+						MinItems: 0,
+						MaxItems: 0,
+					},
+				},
+			}),
+		},
 		"nested attributes and blocks": {
 			map[string]*Schema{
 				"foo": {