cmd/compile: revise error msg in type assertion ptr/non-ptr condition

Revise the error message under the condition that type assertion has
mismatching pointer/non-pointer type.

This adds a better error hint for the situation that a user writes x.(*I),
but they meant to write x.(I).

Fixes #43673

Author: howjmay

src/cmd/compile/internal/gc/typecheck.go

@@ -999,7 +999,12 @@ func typecheck1(n *Node, top int) (res *Node) {
 					yyerror("impossible type assertion:\n\t%v does not implement %v (missing %v method)\n"+
 						"\t\thave %v%0S\n\t\twant %v%0S", n.Type, t, missing.Sym, have.Sym, have.Type, missing.Sym, missing.Type)
 				} else {
-					yyerror("impossible type assertion:\n\t%v does not implement %v (missing %v method)", n.Type, t, missing.Sym)
+					origMissing := missing
+					if implements(derefall(n.Type), t, &missing, &have, &ptr) {
+						yyerror("impossible type assertion:\n\t%v does not implement %v (but %v does)", n.Type, missing.Sym, derefall(n.Type))
+					} else {
+						yyerror("impossible type assertion:\n\t%v does not implement %v (missing %v method)", n.Type, t, origMissing.Sym)
+					}
 				}
 				n.Type = nil
 				return n

test/fixedbugs/issue43673.go

@@ -0,0 +1,21 @@
+// errorcheck
+
+// 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.
+
+// Issue 43673
+package main
+
+type I interface {
+	M()
+}
+
+type T struct{}
+
+func (t *T) M() {}
+
+func main() {
+	var i I
+	_ = i.(*I) // ERROR " *I does not implement M (but I does)"
+}