From 0e919ff2c978294c9b0472055b96bb1a09606934 Mon Sep 17 00:00:00 2001
From: Luuk van Dijk <lvd@golang.org>
Date: Tue, 24 Jan 2012 13:53:00 +0100
Subject: [PATCH] gc: static implements check on typeswitches only applies to
 concrete case types.

Fixes #2700.

R=rsc
CC=golang-dev
https://golang.org/cl/5574046
---
 src/cmd/gc/swt.c    |  2 +-
 test/typeswitch3.go | 27 +++++++++++++++++++++------
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/cmd/gc/swt.c b/src/cmd/gc/swt.c
index 6c0a9ac832f..f1a95587fd6 100644
--- a/src/cmd/gc/swt.c
+++ b/src/cmd/gc/swt.c
@@ -889,7 +889,7 @@ typecheckswitch(Node *n)
 						yyerror("%lN is not a type", ll->n);
 						// reset to original type
 						ll->n = n->ntest->right;
-					} else if(!implements(ll->n->type, t, &missing, &have, &ptr)) {
+					} else if(ll->n->type->etype != TINTER && !implements(ll->n->type, t, &missing, &have, &ptr)) {
 						if(have && !missing->broke && !have->broke)
 							yyerror("impossible type switch case: %lN cannot have dynamic type %T"
 								" (wrong type for %S method)\n\thave %S%hT\n\twant %S%hT",
diff --git a/test/typeswitch3.go b/test/typeswitch3.go
index 99d08a20f77..078980146f8 100644
--- a/test/typeswitch3.go
+++ b/test/typeswitch3.go
@@ -6,15 +6,30 @@
 
 package main
 
+import (
+	"io"
+)
 
 type I interface {
-       M()
+	M()
 }
 
 func main(){
-       var x I
-       switch x.(type) {
-       case string:	// ERROR "impossible"
-               println("FAIL")
-       }
+	var x I
+	switch x.(type) {
+	case string:	// ERROR "impossible"
+		println("FAIL")
+	}
+	
+	// Issue 2700: if the case type is an interface, nothing is impossible
+	
+	var r io.Reader
+	
+	_, _ = r.(io.Writer)
+	
+	switch r.(type) {
+	case io.Writer:
+	}
 }
+
+