cmd/compile, go/*: move method type parameter checks from parsers to type checkers

The parsers (cmd/compile/internal/syntax and go/parser) always accepted
type parameters on methods for parser robustness but reported an error.

With this change, the parsers accept the type parameters on methods,
and then the type checkers (cmd/compile/internal/types2 and go/types)
complain about them.

Add test case for method type parameters when running the parsers only.
Adjust some existing test cases as needed.

This change is a necessary first step towards implementing generic
methods but does not enable them.

For #77273.

Change-Id: I291fcf0aef0c917c74b32131c88b9e4ed71c5060
Reviewed-on: https://go-review.googlesource.com/c/go/+/738441
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

4 files changed, 39 insertions, 5 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index 8278685943..aade7b9fd3 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -797,9 +797,9 @@ func (p *parser) funcDeclOrNil() *FuncDecl {
 	f.pos = p.pos()
 	f.Pragma = p.takePragma()
 
-	var context string
+	hasRecv := false
 	if p.got(_Lparen) {
-		context = "method"
+		hasRecv = true
 		rcvr := p.paramList(nil, nil, _Rparen, false, false)
 		switch len(rcvr) {
 		case 0:
@@ -814,13 +814,13 @@ func (p *parser) funcDeclOrNil() *FuncDecl {
 
 	if p.tok == _Name {
 		f.Name = p.name()
-		f.TParamList, f.Type = p.funcType(context)
+		f.TParamList, f.Type = p.funcType("")
 	} else {
 		f.Name = NewName(p.pos(), "_")
 		f.Type = new(FuncType)
 		f.Type.pos = p.pos()
 		msg := "expected name or ("
-		if context != "" {
+		if hasRecv {
 			msg = "expected name"
 		}
 		p.syntaxError(msg)
diff --git a/src/cmd/compile/internal/syntax/testdata/smoketest.go b/src/cmd/compile/internal/syntax/testdata/smoketest.go
index 6b3593ac7a..e7c9f7283f 100644
--- a/src/cmd/compile/internal/syntax/testdata/smoketest.go
+++ b/src/cmd/compile/internal/syntax/testdata/smoketest.go
@@ -71,3 +71,33 @@ type _ interface {
 	T[P]
 	T[P1, P2]
 }
+
+// generic method
+type List[E any] []E
+
+func (l List[E]) Map[F any](m func(E) F) (r List[F]) {
+	for _, x := range l {
+		r = append(r, m(x))
+	}
+	return
+}
+
+func _() {
+	l := List[string]{"foo", "foobar", "42"}
+	r := l.Map(func(s string) int { return len(s)})
+	_ = r
+}
+
+func _[E, F any](l List[E]) List[F] {
+	var f func(List[E], func(E) F) List[F] = List[E].Map  // method expression & type inference
+	return f(l, func(E) F { var f F; return f })
+}
+
+// disallowed type parameters
+
+type _ func /* ERROR function type must have no type parameters */ [P any](P)
+type _ interface {
+	m /* ERROR interface method must have no type parameters */ [P any](P)
+}
+
+var _ = func /* ERROR function type must have no type parameters */ [P any](P) {}
diff --git a/src/cmd/compile/internal/types2/resolver.go b/src/cmd/compile/internal/types2/resolver.go
index f075f5447f..0911ae9c90 100644
--- a/src/cmd/compile/internal/types2/resolver.go
+++ b/src/cmd/compile/internal/types2/resolver.go
@@ -452,6 +452,11 @@ func (check *Checker) collectObjects() {
 					if recv, _ := base.(*syntax.Name); recv != nil && name != "_" {
 						methods = append(methods, methodInfo{obj, ptr, recv})
 					}
+					// methods cannot have type parameters for now
+					if len(s.TParamList) != 0 {
+						check.softErrorf(s.TParamList[0], InvalidMethodTypeParams, "method %s must have no type parameters", name)
+						hasTParamError = true
+					}
 					check.recordDef(s.Name, obj)
 				}
 				_ = len(s.TParamList) != 0 && !hasTParamError && check.verifyVersionf(s.TParamList[0], go1_18, "type parameter")
diff --git a/src/cmd/compile/internal/types2/signature.go b/src/cmd/compile/internal/types2/signature.go
index d569ba8013..9d739909fd 100644
--- a/src/cmd/compile/internal/types2/signature.go
+++ b/src/cmd/compile/internal/types2/signature.go
@@ -155,7 +155,6 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 
 	// collect and declare function type parameters
 	if tparams != nil {
-		// The parser will complain about invalid type parameters for methods.
 		check.collectTypeParams(&sig.tparams, tparams)
 	}