go/types, types: proceed with correct (invalid) type in case of a selector error

Fixes #76103.

Change-Id: Idc2f5d1d7aeb4a9b468e7c268e3bf5b85d1c3777
Reviewed-on: https://go-review.googlesource.com/c/go/+/716300
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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

@@ -931,6 +931,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *TypeName
 
 Error:
 	x.mode = invalid
+	x.typ = Typ[Invalid]
 	x.expr = e
 }
 

src/go/types/call.go

@@ -979,6 +979,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *TypeName, w
 
 Error:
 	x.mode = invalid
+	x.typ = Typ[Invalid]
 	x.expr = e
 }
 

src/internal/types/testdata/fixedbugs/issue76103.go

@@ -0,0 +1,29 @@
+// Copyright 2025 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
+
+func _() {
+	f(foo /* ERROR "undefined: foo" */) // ERROR "not enough arguments in call to f\n\thave (unknown type)\n\twant (int, int)"
+}
+
+func f(_, _ int) {}
+
+// test case from issue
+
+type S struct{}
+
+func (S) G() {}
+
+func main() {
+	var s S
+	_ = must(s.F /* ERROR "s.F undefined" */ ()) // ERROR "not enough arguments in call to must\n\thave (unknown type)\n\twant (T, error)"
+}
+
+func must[T any](x T, err error) T {
+	if err != nil {
+		panic(err)
+	}
+	return x
+}