gc: select receive with implicit conversion

Fixes #1172.

R=ken2
CC=golang-dev
https://golang.org/cl/2615041

src/cmd/gc/select.c

@@ -41,11 +41,18 @@ typecheckselect(Node *sel)
 			setlineno(n);
 			switch(n->op) {
 			default:
-				yyerror("select case must be receive, send or assign recv");;
+				yyerror("select case must be receive, send or assign recv");
 				break;
 
 			case OAS:
 				// convert x = <-c into OSELRECV(x, c)
+				// assignment might have introduced a
+				// conversion.  throw it away.
+				// it will come back when the select code
+				// gets generated, because it always assigns
+				// through a temporary.
+				if((n->right->op == OCONVNOP || n->right->op == OCONVIFACE) && n->right->implicit)
+					n->right = n->right->left;
 				if(n->right->op != ORECV) {
 					yyerror("select assignment must have receive on right hand side");
 					break;

src/cmd/gc/subr.c

@@ -1046,6 +1046,9 @@ Jconv(Fmt *fp)
 	if(n->isddd != 0)
 		fmtprint(fp, " isddd(%d)", n->isddd);
 
+	if(n->implicit != 0)
+		fmtprint(fp, " implicit(%d)", n->implicit);
+
 	return 0;
 }
 
@@ -2047,6 +2050,7 @@ assignconv(Node *n, Type *t, char *context)
 	r = nod(op, n, N);
 	r->type = t;
 	r->typecheck = 1;
+	r->implicit = 1;
 	return r;
 }
 

test/fixedbugs/bug312.go

@@ -0,0 +1,22 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 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 1172
+
+package main
+
+func main() {
+	var i interface{}
+	c := make(chan int, 1)
+	c <- 1
+	select {
+	case i = <-c: // error on this line
+	}
+	if i != 1 {
+		println("bad i", i)
+		panic("BUG")
+	}
+}