cmd/compile/internal/types2: remove Config.AcceptMethodTypeParams flag

Type parameters for methods are not part of the accepted language,
but maintaining the code for type-checking them ensures regularity
of the type checker implementation. For now, keep the flag internally,
disabled by default. The flag is set when running tests.

Change-Id: Ic99934bd00bd2608dc1178e4131f46dd1507f0f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/307214
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/api.go

@@ -107,9 +107,6 @@ 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/cmd/compile/internal/types2/api_test.go

@@ -65,9 +65,8 @@ func mayTypecheck(t *testing.T, path, source string, info *Info) (string, error)
 		t.Fatalf("%s: unable to parse: %s", path, err)
 	}
 	conf := Config{
-		AcceptMethodTypeParams: true,
-		Error:                  func(err error) {},
-		Importer:               defaultImporter(),
+		Error:    func(err error) {},
+		Importer: defaultImporter(),
 	}
 	pkg, err := conf.Check(f.PkgName.Value, []*syntax.File{f}, info)
 	return pkg.Name(), err

src/cmd/compile/internal/types2/check.go

@@ -31,11 +31,6 @@ const debug = false // leave on during development
 //
 const forceStrict = false
 
-// If methodTypeParamsOk is set, type parameters are
-// permitted in method declarations (in interfaces, too).
-// Generalization and experimental feature.
-const methodTypeParamsOk = true
-
 // exprInfo stores information about an untyped expression.
 type exprInfo struct {
 	isLhs bool // expression is lhs operand of a shift with delayed type-check

src/cmd/compile/internal/types2/check_test.go

@@ -128,7 +128,6 @@ func checkFiles(t *testing.T, filenames []string, goVersion string, colDelta uin
 	// typecheck and collect typechecker errors
 	var conf Config
 	conf.GoVersion = goVersion
-	conf.AcceptMethodTypeParams = true
 	// special case for importC.src
 	if len(filenames) == 1 && strings.HasSuffix(filenames[0], "importC.src") {
 		conf.FakeImportC = true

src/cmd/compile/internal/types2/resolver.go

@@ -426,7 +426,7 @@ func (check *Checker) collectObjects() {
 				} else {
 					// method
 					// d.Recv != nil
-					if !methodTypeParamsOk && len(d.TParamList) != 0 {
+					if !acceptMethodTypeParams && len(d.TParamList) != 0 {
 						//check.error(d.TParamList.Pos(), invalidAST + "method must have no type parameters")
 						check.error(d, invalidAST+"method must have no type parameters")
 					}

src/cmd/compile/internal/types2/types_test.go

@@ -0,0 +1,9 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types2
+
+func init() {
+	acceptMethodTypeParams = true
+}

src/cmd/compile/internal/types2/typexpr.go

@@ -15,6 +15,9 @@ import (
 	"strings"
 )
 
+// Disabled by default, but enabled when running tests (via types_test.go).
+var acceptMethodTypeParams bool
+
 // ident type-checks identifier e and initializes x with the value or type of e.
 // If an error occurred, x.mode is set to invalid.
 // For the meaning of def, see Checker.definedType, below.
@@ -336,7 +339,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, 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 {
+		if recvPar != nil && !acceptMethodTypeParams {
 			check.error(ftyp, "methods cannot have type parameters")
 		}
 	}
@@ -848,7 +851,7 @@ func (check *Checker) interfaceType(ityp *Interface, iface *syntax.InterfaceType
 			// 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 {
+			if sig.tparams != nil && !acceptMethodTypeParams {
 				check.error(f.Type, "methods cannot have type parameters")
 			}