cmd/internal/gc: type of str[i] is byte, not uint8

Fixes #8745.

Change-Id: Id0641e3c0f259812b41ed871e83c68740feb2b19
Reviewed-on: https://go-review.googlesource.com/10261Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/gc/typecheck.go

@@ -1027,7 +1027,7 @@ OpSwitch:
 		case TSTRING, TARRAY:
 			indexlit(&n.Right)
 			if t.Etype == TSTRING {
-				n.Type = Types[TUINT8]
+				n.Type = bytetype
 			} else {
 				n.Type = t.Type
 			}

test/fixedbugs/issue8745.go

+// errorcheck
+
+// Copyright 2015 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.
+
+// Check that the error says s[2] is a byte, not a uint8.
+
+package p
+
+func f(s string) {
+	var _ float64 = s[2] // ERROR "cannot use.*type byte.*as type float64"
+}