go/parser: check presence of 2nd and 3rd index in 3-index slice

Fixes #7305.

LGTM=adonovan
R=bradfitz, adonovan
CC=golang-codereviews
https://golang.org/cl/58950045

src/pkg/go/parser/parser.go

@@ -1168,16 +1168,19 @@ func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 		defer un(trace(p, "IndexOrSlice"))
 	}
 
+	const N = 3 // change the 3 to 2 to disable 3-index slices
 	lbrack := p.expect(token.LBRACK)
 	p.exprLev++
-	var index [3]ast.Expr // change the 3 to 2 to disable slice expressions w/ cap
+	var index [N]ast.Expr
+	var colons [N - 1]token.Pos
 	if p.tok != token.COLON {
 		index[0] = p.parseRhs()
 	}
 	ncolons := 0
-	for p.tok == token.COLON && ncolons < len(index)-1 {
-		p.next()
+	for p.tok == token.COLON && ncolons < len(colons) {
+		colons[ncolons] = p.pos
 		ncolons++
+		p.next()
 		if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
 			index[ncolons] = p.parseRhs()
 		}
@@ -1187,7 +1190,21 @@ func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 
 	if ncolons > 0 {
 		// slice expression
-		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: ncolons == 2, Rbrack: rbrack}
+		slice3 := false
+		if ncolons == 2 {
+			slice3 = true
+			// Check presence of 2nd and 3rd 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")
+				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")
+				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
+			}
+		}
+		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
 	}
 
 	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}

src/pkg/go/parser/short_test.go

@@ -76,7 +76,11 @@ 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[::: /* ERROR "expected ']'" */ ] };`,
+	`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:k: /* ERROR "expected ']'" */ l] };`,
 }