gc: avoid meaningless constant overflow error for inverted slice range

Used to say:

issue4251.go:12: inverted slice range
issue4251.go:12: constant -1 overflows uint64
issue4251.go:16: inverted slice range
issue4251.go:16: constant -1 overflows uint64
issue4251.go:20: inverted slice range
issue4251.go:20: constant -1 overflows uint64

With this patch, only gives the "inverted slice range" errors.

R=golang-dev, daniel.morsing
CC=golang-dev
https://golang.org/cl/6871058

src/cmd/gc/typecheck.c

@@ -950,12 +950,16 @@ reswitch:
 				goto error;
 			}
 			if(n->right->left->op == OLITERAL) {
-				if(mpgetfix(n->right->left->val.u.xval) < 0)
+				if(mpgetfix(n->right->left->val.u.xval) < 0) {
 					yyerror("invalid slice index %N (index must be non-negative)", n->right->left);
-				else if(tp != nil && tp->bound > 0 && mpgetfix(n->right->left->val.u.xval) > tp->bound)
+					goto error;
+				} else if(tp != nil && tp->bound > 0 && mpgetfix(n->right->left->val.u.xval) > tp->bound) {
 					yyerror("invalid slice index %N (out of bounds for %d-element array)", n->right->left, tp->bound);
-				else if(mpcmpfixfix(n->right->left->val.u.xval, maxintval[TINT]) > 0)
+					goto error;
+				} else if(mpcmpfixfix(n->right->left->val.u.xval, maxintval[TINT]) > 0) {
 					yyerror("invalid slice index %N (index too large)", n->right->left);
+					goto error;
+				}
 			}
 		}
 		if(n->right->right != N) {
@@ -966,14 +970,26 @@ reswitch:
 				goto error;
 			}
 			if(n->right->right->op == OLITERAL) {
-				if(mpgetfix(n->right->right->val.u.xval) < 0)
+				if(mpgetfix(n->right->right->val.u.xval) < 0) {
 					yyerror("invalid slice index %N (index must be non-negative)", n->right->right);
-				else if(tp != nil && tp->bound > 0 && mpgetfix(n->right->right->val.u.xval) > tp->bound)
+					goto error;
+				} else if(tp != nil && tp->bound > 0 && mpgetfix(n->right->right->val.u.xval) > tp->bound) {
 					yyerror("invalid slice index %N (out of bounds for %d-element array)", n->right->right, tp->bound);
-				else if(mpcmpfixfix(n->right->right->val.u.xval, maxintval[TINT]) > 0)
+					goto error;
+				} else if(mpcmpfixfix(n->right->right->val.u.xval, maxintval[TINT]) > 0) {
 					yyerror("invalid slice index %N (index too large)", n->right->right);
+					goto error;
+				}
 			}
 		}
+		if(n->right->left != N
+		   && n->right->right != N
+		   && n->right->left->op == OLITERAL
+		   && n->right->right->op == OLITERAL
+		   && mpcmpfixfix(n->right->left->val.u.xval, n->right->right->val.u.xval) > 0) {
+			yyerror("inverted slice index %N > %N", n->right->left, n->right->right);
+			goto error;
+		}
 		goto ret;
 
 	/*

src/cmd/gc/walk.c

@@ -2517,8 +2517,6 @@ sliceany(Node* n, NodeList **init)
 		else
 			bv = mpgetfix(bound->val.u.xval);
 	}
-	lbv = -1;
-	hbv = -1;
 
 	if(isconst(hb, CTINT)) {
 		hbv = mpgetfix(hb->val.u.xval);
@@ -2536,8 +2534,6 @@ sliceany(Node* n, NodeList **init)
 		if(lbv == 0)
 			lb = N;
 	}
-	if(lbv >= 0 && hbv >= 0 && lbv > hbv)
-		yyerror("inverted slice range");
 
 	// dynamic checks convert all bounds to unsigned to save us the bound < 0 comparison
 	// generate

test/fixedbugs/issue4251.go

+// 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 4251: slice with inverted range is an error.
+
+package p
+
+func F1(s []byte) []byte {
+	return s[2:1]		// ERROR "inverted"
+}
+
+func F2(a [10]byte) []byte {
+	return a[2:1]		// ERROR "inverted"
+}
+
+func F3(s string) string {
+	return s[2:1]		// ERROR "inverted"
+}