go/types: better position for invalid slice indices error

This is a port of CL 363671 from types2 to go/types. Also adjust the
error message to match types2 ("invalid" vs "swapped").

Change-Id: I662a73c915814fea14bfcb1ebde0fbf39589f022
Reviewed-on: https://go-review.googlesource.com/c/go/+/364902
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

src/go/types/index.go

@@ -310,9 +310,13 @@ func (check *Checker) sliceExpr(x *operand, e *ast.SliceExpr) {
 L:
 	for i, x := range ind[:len(ind)-1] {
 		if x > 0 {
-			for _, y := range ind[i+1:] {
-				if y >= 0 && x > y {
-					check.errorf(inNode(e, e.Rbrack), _SwappedSliceIndices, "swapped slice indices: %d > %d", x, y)
+			for j, y := range ind[i+1:] {
+				if y >= 0 && y < x {
+					// The value y corresponds to the expression e.Index[i+1+j].
+					// Because y >= 0, it must have been set from the expression
+					// when checking indices and thus e.Index[i+1+j] is not nil.
+					at := []ast.Expr{e.Low, e.High, e.Max}[i+1+j]
+					check.errorf(at, _SwappedSliceIndices, "invalid slice indices: %d < %d", y, x)
 					break L // only report one error, ok to continue
 				}
 			}

src/go/types/testdata/check/expr3.src

@@ -44,9 +44,9 @@ func indexes() {
 	_ = a[:10:10]
 	_ = a[:11 /* ERROR "index .* out of bounds" */ :10]
 	_ = a[:10:11 /* ERROR "index .* out of bounds" */ ]
-	_ = a[10:0:10] /* ERROR swapped slice indices" */
-	_ = a[0:10:0] /* ERROR "swapped slice indices" */
-	_ = a[10:0:0] /* ERROR "swapped slice indices" */
+	_ = a[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = a[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = a[10:0 /* ERROR "invalid slice indices" */:0]
 	_ = &a /* ERROR "cannot take address" */ [:10]
 
 	pa := &a
@@ -62,9 +62,9 @@ func indexes() {
 	_ = pa[:10:10]
 	_ = pa[:11 /* ERROR "index .* out of bounds" */ :10]
 	_ = pa[:10:11 /* ERROR "index .* out of bounds" */ ]
-	_ = pa[10:0:10] /* ERROR "swapped slice indices" */
-	_ = pa[0:10:0] /* ERROR "swapped slice indices" */
-	_ = pa[10:0:0] /* ERROR "swapped slice indices" */
+	_ = pa[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = pa[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = pa[10:0 /* ERROR "invalid slice indices" */ :0]
 	_ = &pa /* ERROR "cannot take address" */ [:10]
 
 	var b [0]int
@@ -82,16 +82,16 @@ func indexes() {
 	_ = s[: - /* ERROR "negative" */ 1]
 	_ = s[0]
 	_ = s[1:2]
-	_ = s[2:1] /* ERROR "swapped slice indices" */
+	_ = s[2:1 /* ERROR "invalid slice indices" */ ]
 	_ = s[2:]
 	_ = s[: 1 /* ERROR "overflows" */ <<100]
 	_ = s[1 /* ERROR "overflows" */ <<100 :]
 	_ = s[1 /* ERROR "overflows" */ <<100 : 1 /* ERROR "overflows" */ <<100]
 	_ = s[: /* ERROR "2nd index required" */ :  /* ERROR "3rd index required" */ ]
 	_ = s[:10:10]
-	_ = s[10:0:10] /* ERROR "swapped slice indices" */
-	_ = s[0:10:0] /* ERROR "swapped slice indices" */
-	_ = s[10:0:0] /* ERROR "swapped slice indices" */
+	_ = s[10:0 /* ERROR "invalid slice indices" */ :10]
+	_ = s[0:10:0 /* ERROR "invalid slice indices" */ ]
+	_ = s[10:0 /* ERROR "invalid slice indices" */ :0]
 	_ = &s /* ERROR "cannot take address" */ [:10]
 
 	var m map[string]int