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.

close #43673

Author: howjmay

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

@@ -999,7 +999,13 @@ 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)
+					var _missing, _have *types.Field
+					var _ptr int
+					if implements(derefall(n.Type), t, &_missing, &_have, &_ptr) {
+						yyerror("impossible type assertion:\n\t instead of type %v need to type assert to %v", n.Type, derefall(n.Type))
+					} else {
+						yyerror("impossible type assertion:\n\t%v does not implement %v (missing %v method)", n.Type, t, missing.Sym)
+					}
 				}
 				n.Type = nil
 				return n