From 6a41d32189ed90822cf9d6d9d5bb8a67f3531c17 Mon Sep 17 00:00:00 2001
From: HowJMay <vulxj0j8j8@gmail.com>
Date: Fri, 22 Jan 2021 01:12:51 +0800
Subject: [PATCH] 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
---
 src/cmd/compile/internal/gc/typecheck.go |  7 ++++++-
 test/fixedbugs/issue43831.go             | 21 +++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 test/fixedbugs/issue43831.go

diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index c0b05035f033c1..a12caaa42be550 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/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 method (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
diff --git a/test/fixedbugs/issue43831.go b/test/fixedbugs/issue43831.go
new file mode 100644
index 00000000000000..fabc687e2435b5
--- /dev/null
+++ b/test/fixedbugs/issue43831.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 43831
+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 method (but I does)"
+}