go/types, types2: better error when selecting field on type rather than value

Fixes #6814.

Change-Id: I659670998f8e89400d03d40189e8c54f7e705cdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/738040
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

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

@@ -835,6 +835,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, wantType bool
 		check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (%s)", x.expr, sel, why)
 		goto Error
 	}
+	// obj != nil
 
 	// methods may not have a fully set up signature yet
 	if m, _ := obj.(*Func); m != nil {
@@ -845,7 +846,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, wantType bool
 		// method expression
 		m, _ := obj.(*Func)
 		if m == nil {
-			check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
+			check.errorf(e.X, MissingFieldOrMethod, "operand for field selector %s must be value of type %s", sel, x.typ)
 			goto Error
 		}
 

src/go/types/call.go

@@ -838,6 +838,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, wantType bool) {
 		check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (%s)", x.expr, sel, why)
 		goto Error
 	}
+	// obj != nil
 
 	// methods may not have a fully set up signature yet
 	if m, _ := obj.(*Func); m != nil {
@@ -848,7 +849,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, wantType bool) {
 		// method expression
 		m, _ := obj.(*Func)
 		if m == nil {
-			check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
+			check.errorf(e.X, MissingFieldOrMethod, "operand for field selector %s must be value of type %s", sel, x.typ)
 			goto Error
 		}
 

src/internal/types/testdata/check/expr3.go

@@ -156,7 +156,7 @@ func (*T) m() {}
 
 func method_expressions() {
 	_ = T.a /* ERROR "no field or method" */
-	_ = T.x /* ERROR "has no method" */
+	_ = T /* ERROR "operand for field selector x must be value of type T" */ .x
 	_ = T.m /* ERROR "invalid method expression T.m (needs pointer receiver (*T).m)" */
 	_ = (*T).m
 
@@ -164,8 +164,8 @@ func method_expressions() {
 	var g func(*T) = (*T).m
 	_, _ = f, g
 
-	_ = T.y /* ERROR "has no method" */
-	_ = (*T).y /* ERROR "has no method" */
+	_ = T /* ERROR "operand for field selector y must be value of type T" */ .y
+	_ = ( /* ERROR "operand for field selector y must be value of type *T" */ *T).y
 }
 
 func struct_literals() {

src/internal/types/testdata/check/issues0.go

@@ -351,7 +351,7 @@ func issue26234b(x T) {
 }
 
 func issue26234c() {
-	T.x /* ERROR "T.x undefined (type T has no method x)" */ ()
+	T /* ERROR "operand for field selector x must be value of type T" */ .x()
 }
 
 func issue35895() {

src/internal/types/testdata/fixedbugs/issue6814.go

@@ -0,0 +1,35 @@
+// Copyright 2026 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 p
+
+type E struct {
+	e int
+}
+
+func (E) m() {}
+
+type S struct {
+	E
+	x int
+}
+
+func (S) n() {}
+
+func _() {
+	_ = S.X // ERROR "S.X undefined (type S has no field or method X, but does have field x)"
+	_ = S /* ERROR "operand for field selector E must be value of type S" */ .E
+	_ = S /* ERROR "operand for field selector x must be value of type S" */ .x
+	_ = S /* ERROR "operand for field selector e must be value of type S" */ .e
+	_ = S.m
+	_ = S.n
+
+	var s S
+	_ = s.X // ERROR "s.X undefined (type S has no field or method X, but does have field x)"
+	_ = s.E
+	_ = s.x
+	_ = s.e
+	_ = s.m
+	_ = s.n
+}