cmd/cue: Go types implementing a supported interface are themselves supported

Otherwise, the tweaked test case type, which implements yaml.Marshaler,
would get discarded as an unsupported struct given that it's only
made up of unexported fields.

Note that the existing CustomYAML test case was buggy;
it did not actually use the correct MarshalYAML method signature,
so we weren't testing the fact that it was a custom yaml.Marshaler.
This is now fixed, which causes it to generate as "top" as expected.

Fixes #3495.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ie9926daf829e21c9e9f117f1a813edcd9b6f50d3
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1224699
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

cmd/cue/cmd/get_go.go

@@ -732,7 +732,7 @@ func (e *extractor) reportDecl(x *ast.GenDecl) (a []cueast.Decl) {
 				fallthrough
 
 			default:
-				if !supportedType(nil, typ) {
+				if !e.supportedType(nil, typ) {
 					e.logf("    Dropped declaration %v of unsupported type %v", name, typ)
 					continue
 				}
@@ -976,7 +976,11 @@ func makeDoc(g *ast.CommentGroup, isDoc bool) *cueast.CommentGroup {
 	return &cueast.CommentGroup{Doc: isDoc, List: a}
 }
 
-func supportedType(stack []types.Type, t types.Type) (ok bool) {
+func (e *extractor) supportedType(stack []types.Type, t types.Type) (ok bool) {
+	if s := e.altType(t); s != nil {
+		// t implements a supported interface.
+		return true
+	}
 	// handle recursive types
 	if slices.Contains(stack, t) {
 		return true
@@ -1015,27 +1019,27 @@ func supportedType(stack []types.Type, t types.Type) (ok bool) {
 	case *types.Basic:
 		return true
 	case *types.Named:
-		return supportedType(stack, t.Underlying())
+		return e.supportedType(stack, t.Underlying())
 	case *types.TypeParam:
-		return supportedType(stack, t.Underlying())
+		return e.supportedType(stack, t.Underlying())
 	case *types.Pointer:
-		return supportedType(stack, t.Elem())
+		return e.supportedType(stack, t.Elem())
 	case *types.Slice:
-		return supportedType(stack, t.Elem())
+		return e.supportedType(stack, t.Elem())
 	case *types.Array:
-		return supportedType(stack, t.Elem())
+		return e.supportedType(stack, t.Elem())
 	case *types.Map:
 		if b, ok := t.Key().Underlying().(*types.Basic); !ok || b.Kind() != types.String {
 			return false
 		}
-		return supportedType(stack, t.Elem())
+		return e.supportedType(stack, t.Elem())
 	case *types.Struct:
 		// Eliminate structs with fields for which all fields are filtered.
 		if t.NumFields() == 0 {
 			return true
 		}
 		for f := range t.Fields() {
-			if f.Exported() && supportedType(stack, f.Type()) {
+			if f.Exported() && e.supportedType(stack, f.Type()) {
 				return true
 			}
 		}
@@ -1365,7 +1369,7 @@ func (e *extractor) addFields(x *types.Struct, st *cueast.StructLit) {
 		if !ast.IsExported(f.Name()) {
 			continue
 		}
-		if !supportedType(nil, f.Type()) {
+		if !e.supportedType(nil, f.Type()) {
 			e.logf("    Dropped field %v for unsupported type %v", f.Name(), f.Type())
 			continue
 		}

cmd/cue/cmd/testdata/script/get_go_types.txtar

@@ -215,9 +215,10 @@ func (c *CustomJSON) MarshalJSON() ([]byte, error) {
 }
 
 type CustomYAML struct {
+	unexported int
 }
 
-func (c CustomYAML) MarshalYAML() ([]byte, error) {
+func (c CustomYAML) MarshalYAML() (any, error) {
 	return nil, nil
 }
 
@@ -479,7 +480,7 @@ _#internalIdentifier: #Identifier & "internal"
 
 #CustomJSON: _
 
-#CustomYAML: {}
+#CustomYAML: _
 
 _#localType: int