go/types, types2: disallow new methods on (aliases to) cgo-generated types

This makes methods on aliases of cgo-generated types a new compiler error.
That is ok because cgo-behavior is not covered by the G1 compatibility
guarantee.

Background: In 2023 we fixed a gopls issue related to this by actually
enabling methods on cgo-generated types in the first place (#59944).
See the discussion in #60725 and this CL for why we believe it is ok
to make this an error now.

Based on a variation of CL 503596 (by Xie Cui).

Fixes #60725.
For #59944.

Change-Id: I7e9e6e1a76447167483a282b268f5183214027c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/629715
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: griesemer

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

@@ -839,7 +839,8 @@ func TestIssue59944(t *testing.T) {
 	testenv.MustHaveCGO(t)
 
 	// The typechecker should resolve methods declared on aliases of cgo types.
-	const src = `
+	const src = `// -gotypesalias=1
+
 package p
 
 /*
@@ -851,7 +852,7 @@ import "C"
 
 type Layout = C.struct_layout
 
-func (l *Layout) Binding() {}
+func (l /* ERROR "cannot define new methods on non-local type Layout" */ *Layout) Binding() {}
 
 func _() {
 	_ = (*Layout).Binding

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

@@ -8,6 +8,8 @@ import (
 	"cmd/compile/internal/syntax"
 	"fmt"
 	. "internal/types/errors"
+	"path/filepath"
+	"strings"
 )
 
 // ----------------------------------------------------------------------------
@@ -410,7 +412,7 @@ func (check *Checker) validRecv(recv *Var) {
 	// as the method."
 	switch T := atyp.(type) {
 	case *Named:
-		if T.obj.pkg != check.pkg {
+		if T.obj.pkg != check.pkg || isCGoTypeObj(T.obj) {
 			check.errorf(recv, InvalidRecv, "cannot define new methods on non-local type %s", rtyp)
 			break
 		}
@@ -437,3 +439,9 @@ func (check *Checker) validRecv(recv *Var) {
 		check.errorf(recv, InvalidRecv, "invalid receiver type %s", recv.typ)
 	}
 }
+
+// isCGoTypeObj reports whether the given type name was created by cgo.
+func isCGoTypeObj(obj *TypeName) bool {
+	return strings.HasPrefix(obj.name, "_Ctype_") ||
+		strings.HasPrefix(filepath.Base(obj.pos.FileBase().Filename()), "_cgo_")
+}

src/cmd/go/testdata/script/cgo_badmethod_issue60725.txt

@@ -0,0 +1,26 @@
+[short] skip
+[!cgo] skip
+
+# Test that cgo rejects attempts to declare methods
+# on the types A or *A; see issue #60725.
+
+! go build ./a
+stderr 'cannot define new methods on non-local type A'
+stderr 'cannot define new methods on non-local type A'
+
+-- go.mod --
+module example.com
+go 1.24
+
+-- a/a.go --
+package a
+
+/*
+typedef int T;
+*/
+import "C"
+
+type A = C.T
+
+func (A) m1() {}
+func (*A) m2() {}

src/go/types/issues_test.go

@@ -848,7 +848,8 @@ func TestIssue59944(t *testing.T) {
 	testenv.MustHaveCGO(t)
 
 	// The typechecker should resolve methods declared on aliases of cgo types.
-	const src = `
+	const src = `// -gotypesalias=1
+
 package p
 
 /*
@@ -860,7 +861,7 @@ import "C"
 
 type Layout = C.struct_layout
 
-func (l *Layout) Binding() {}
+func (l /* ERROR "cannot define new methods on non-local type Layout" */ *Layout) Binding() {}
 
 func _() {
 	_ = (*Layout).Binding

src/go/types/signature.go

@@ -9,6 +9,8 @@ import (
 	"go/ast"
 	"go/token"
 	. "internal/types/errors"
+	"path/filepath"
+	"strings"
 )
 
 // ----------------------------------------------------------------------------
@@ -430,7 +432,7 @@ func (check *Checker) validRecv(recv *Var) {
 	// as the method."
 	switch T := atyp.(type) {
 	case *Named:
-		if T.obj.pkg != check.pkg {
+		if T.obj.pkg != check.pkg || isCGoTypeObj(check.fset, T.obj) {
 			check.errorf(recv, InvalidRecv, "cannot define new methods on non-local type %s", rtyp)
 			break
 		}
@@ -457,3 +459,9 @@ func (check *Checker) validRecv(recv *Var) {
 		check.errorf(recv, InvalidRecv, "invalid receiver type %s", recv.typ)
 	}
 }
+
+// isCGoTypeObj reports whether the given type name was created by cgo.
+func isCGoTypeObj(fset *token.FileSet, obj *TypeName) bool {
+	return strings.HasPrefix(obj.name, "_Ctype_") ||
+		strings.HasPrefix(filepath.Base(fset.File(obj.pos).Name()), "_cgo_")
+}