cmd/gc: clean up string index errors

Unify with array/slice errors, which were already good.

Fixes #4232.

R=ken2
CC=golang-dev
https://golang.org/cl/7271046

src/cmd/gc/typecheck.c

@@ -832,24 +832,35 @@ reswitch:
 			yyerror("invalid operation: %N (index of type %T)", n, t);
 			goto error;
 
+
+		case TSTRING:
 		case TARRAY:
 			defaultlit(&n->right, T);
+			if(t->etype == TSTRING)
+				n->type = types[TUINT8];
+			else
 				n->type = t->type;
+			why = "string";
+			if(t->etype == TARRAY) {
+				if(isfixedarray(t))
+					why = "array";
+				else
+					why = "slice";
+			}
 			if(n->right->type != T && !isint[n->right->type->etype]) {
-				yyerror("non-integer array index %N", n->right);
+				yyerror("non-integer %s index %N", why, n->right);
 				break;
 			}
 			if(n->right->op == OLITERAL) {
-			       	if(mpgetfix(n->right->val.u.xval) < 0) {
-					why = isfixedarray(t) ? "array" : "slice";
+			       	if(mpgetfix(n->right->val.u.xval) < 0)
 					yyerror("invalid %s index %N (index must be non-negative)", why, n->right);
-				} else if(isfixedarray(t) && t->bound > 0 && mpgetfix(n->right->val.u.xval) >= t->bound)
+				else if(isfixedarray(t) && t->bound > 0 && mpgetfix(n->right->val.u.xval) >= t->bound)
 					yyerror("invalid array index %N (out of bounds for %d-element array)", n->right, t->bound);
-				else if(mpcmpfixfix(n->right->val.u.xval, maxintval[TINT]) > 0) {
-					why = isfixedarray(t) ? "array" : "slice";
+				else if(isconst(n->left, CTSTR) && mpgetfix(n->right->val.u.xval) >= n->left->val.u.sval->len)
+					yyerror("invalid string index %N (out of bounds for %d-byte string)", n->right, n->left->val.u.sval->len);
+				else if(mpcmpfixfix(n->right->val.u.xval, maxintval[TINT]) > 0)
 					yyerror("invalid %s index %N (index too large)", why, n->right);
 			}
-			}
 			break;
 
 		case TMAP:
@@ -860,13 +871,6 @@ reswitch:
 			n->type = t->type;
 			n->op = OINDEXMAP;
 			break;
-
-		case TSTRING:
-			defaultlit(&n->right, types[TUINT]);
-			if(n->right->type != T && !isint[n->right->type->etype])
-				yyerror("non-integer string index %N", n->right);
-			n->type = types[TUINT8];
-			break;
 		}
 		goto ret;
 

test/fixedbugs/bug205.go

@@ -11,8 +11,8 @@ var s string;
 var m map[string]int;
 
 func main() {
-	println(t["hi"]);	// ERROR "integer"
-	println(s["hi"]);	// ERROR "integer" "to type uint"
-	println(m[0]);	// ERROR "map index"
+	println(t["hi"]);	// ERROR "non-integer slice index"
+	println(s["hi"]);	// ERROR "non-integer string index"
+	println(m[0]);	// ERROR "as type string in map index"
 }
 

test/fixedbugs/issue4232.go

+// errorcheck
+
+// Copyright 2013 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.
+
+package p
+
+func f() {
+	var a [10]int
+	_ = a[-1] // ERROR "invalid array index -1"
+	_ = a[-1:] // ERROR "invalid slice index -1"
+	_ = a[:-1] // ERROR "invalid slice index -1"
+	_ = a[10] // ERROR "invalid array index 10"
+
+	var s []int
+	_ = s[-1] // ERROR "invalid slice index -1"
+	_ = s[-1:] // ERROR "invalid slice index -1"
+	_ = s[:-1] // ERROR "invalid slice index -1"
+	_ = s[10]
+
+	const c = "foo"
+	_ = c[-1] // ERROR "invalid string index -1"
+	_ = c[-1:] // ERROR "invalid slice index -1"
+	_ = c[:-1] // ERROR "invalid slice index -1"
+	_ = c[3] // ERROR "invalid string index 3"
+
+	var t string
+	_ = t[-1] // ERROR "invalid string index -1"
+	_ = t[-1:] // ERROR "invalid slice index -1"
+	_ = t[:-1] // ERROR "invalid slice index -1"
+	_ = t[3]
+}