go/types: match types2 errors for missing index expressions

Use "middle" and "final" rather than "2nd" and "3rd" in error messages
for invalid slice expressions. This is the original compiler error
message and many tests check for this specific message.

For #54511.

Change-Id: I86eb739aa7218b7f393fab1ab402732cb9e9a1f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/424906
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/go/parser/parser.go

@@ -1511,14 +1511,14 @@ func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
 		slice3 := false
 		if ncolons == 2 {
 			slice3 = true
-			// Check presence of 2nd and 3rd index here rather than during type-checking
+			// Check presence of middle and final index here rather than during type-checking
 			// to prevent erroneous programs from passing through gofmt (was issue 7305).
 			if index[1] == nil {
-				p.error(colons[0], "2nd index required in 3-index slice")
+				p.error(colons[0], "middle index required in 3-index slice")
 				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
 			}
 			if index[2] == nil {
-				p.error(colons[1], "3rd index required in 3-index slice")
+				p.error(colons[1], "final index required in 3-index slice")
 				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
 			}
 		}

src/go/parser/short_test.go

@@ -160,11 +160,11 @@ var invalids = []string{
 	`package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
 	`package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,
 	`package p; func f() { var s []int; _ = s[] /* ERROR "expected operand" */ };`,
-	`package p; func f() { var s []int; _ = s[i:j: /* ERROR "3rd index required" */ ] };`,
-	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :k] };`,
-	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :] };`,
-	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ :] };`,
-	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ ::] };`,
+	`package p; func f() { var s []int; _ = s[i:j: /* ERROR "final index required" */ ] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "middle index required" */ :k] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "middle index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "middle index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "middle index required" */ ::] };`,
 	`package p; func f() { var s []int; _ = s[i:j:k: /* ERROR "expected ']'" */ l] };`,
 	`package p; func f() { for x /* ERROR "boolean or range expression" */ = []string {} }`,
 	`package p; func f() { for x /* ERROR "boolean or range expression" */ := []string {} }`,

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

@@ -19,9 +19,9 @@ func indexes() {
 	_ = a[- /* ERROR "negative" */ 1]
 	_ = a[- /* ERROR "negative" */ 1 :]
 	_ = a[: - /* ERROR "negative" */ 1]
-	_ = a[: /* ERROR "2nd index required" */ : /* ERROR "3rd index required" */ ]
-	_ = a[0: /* ERROR "2nd index required" */ : /* ERROR "3rd index required" */ ]
-	_ = a[0: /* ERROR "2nd index required" */ :10]
+	_ = a[: /* ERROR "middle index required" */ : /* ERROR "final index required" */ ]
+	_ = a[0: /* ERROR "middle index required" */ : /* ERROR "final index required" */ ]
+	_ = a[0: /* ERROR "middle index required" */ :10]
 	_ = a[:10:10]
 
 	var a0 int
@@ -87,7 +87,7 @@ func indexes() {
 	_ = 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[: /* ERROR "middle index required" */ :  /* ERROR "final index required" */ ]
 	_ = s[:10:10]
 	_ = s[10:0 /* ERROR "invalid slice indices" */ :10]
 	_ = s[0:10:0 /* ERROR "invalid slice indices" */ ]