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

@@ -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",

test/typeswitch3.go

@@ -6,6 +6,9 @@
 
 package main
 
+import (
+	"io"
+)
 
 type I interface {
 	M()
@@ -17,4 +20,16 @@ func main(){
 	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:
 	}
+}
+
+