go/types: add Config flag to enable method type parameters

The type-checker type-checks method type parameters like
function type parameters without any extra work. But the
current design draft does not permit them and the go2go
translator does not expect them.

Add a flag to enable them for type checker tests, but do
not accept them by default. Instead, type-check but report
an error.

Passes all.bash.

Change-Id: Iec02f4a951a3faccd2409d97d48bc4dd4536a750
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go2-dev/+/764012
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/types/api.go

@@ -107,6 +107,9 @@ type Config struct {
 	// type-checked.
 	IgnoreFuncBodies bool
 
+	// If AcceptMethodTypeParams is set, methods may have type parameters.
+	AcceptMethodTypeParams bool
+
 	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
 	// declares an empty "C" package and errors are omitted for qualified
 	// identifiers referring to package C (which won't find an object).

src/go/types/api_test.go

@@ -49,8 +49,9 @@ func mayTypecheck(t *testing.T, path, source string, info *Info) (string, error)
 		t.Fatalf("%s: unable to parse: %s", path, err)
 	}
 	conf := Config{
-		Error:    func(err error) {},
-		Importer: importer.Default(),
+		AcceptMethodTypeParams: true,
+		Error:                  func(err error) {},
+		Importer:               importer.Default(),
 	}
 	pkg, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, info)
 	return pkg.Name(), err

src/go/types/check_test.go

@@ -281,6 +281,7 @@ func checkFiles(t *testing.T, testfiles []string) {
 
 	// typecheck and collect typechecker errors
 	var conf Config
+	conf.AcceptMethodTypeParams = true
 	// special case for importC.src
 	if len(testfiles) == 1 && testfiles[0] == "testdata/importC.src" {
 		conf.FakeImportC = true

src/go/types/typexpr.go

@@ -311,6 +311,12 @@ func (check *Checker) funcType(sig *Signature, recvPar *ast.FieldList, ftyp *ast
 
 	if ftyp.TParams != nil {
 		sig.tparams = check.collectTypeParams(ftyp.TParams)
+		// Always type-check method type parameters but complain if they are not enabled.
+		// (A separate check is needed when type-checking interface method signatures because
+		// they don't have a receiver specification.)
+		if recvPar != nil && !check.conf.AcceptMethodTypeParams {
+			check.errorf(ftyp.TParams.Pos(), "methods cannot have type parameters")
+		}
 	}
 
 	// Value (non-type) parameters' scope starts in the function body. Use a temporary scope for their
@@ -749,6 +755,13 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 				continue // ignore
 			}
 
+			// Always type-check method type parameters but complain if they are not enabled.
+			// (This extra check is needed here because interface method signatures don't have
+			// a receiver specification.)
+			if sig.tparams != nil && !check.conf.AcceptMethodTypeParams {
+				check.errorf(f.Type.(*ast.FuncType).TParams.Pos(), "methods cannot have type parameters")
+			}
+
 			// use named receiver type if available (for better error messages)
 			var recvTyp Type = ityp
 			if def != nil {