cmd/gc: fix defaultlit of shifts used in interface context.

Fixes #4545.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6937058

src/cmd/gc/const.c

@@ -1052,6 +1052,11 @@ defaultlit(Node **np, Type *t)
 		// When compiling x := 1<<i + 3.14, this means we try to push
 		// the float64 down into the 1<<i, producing the correct error
 		// (cannot shift float64).
+		//
+		// If t is an interface type, we want the default type for the
+		// value, so just do as if no type was given.
+		if(t && t->etype == TINTER)
+			t = T;
 		if(t == T && (n->right->op == OLSH || n->right->op == ORSH)) {
 			defaultlit(&n->left, T);
 			defaultlit(&n->right, n->left->type);

test/fixedbugs/issue4545.go

@@ -0,0 +1,19 @@
+// errorcheck
+
+// Copyright 2012 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 4545: untyped constants are incorrectly coerced
+// to concrete types when used in interface{} context.
+
+package main
+
+import "fmt"
+
+func main() {
+	var s uint
+	fmt.Println(1.0 + 1<<s) // ERROR "invalid operation"
+	x := 1.0 + 1<<s         // ERROR "invalid operation"
+	_ = x
+}