go/parser: accept optional indices for all parts of an index expression

Instead, leave the error testing to the type checker, eventually.

Fixes #5827.

R=adonovan
CC=golang-dev
https://golang.org/cl/10917043

src/pkg/go/parser/parser.go

@@ -1187,10 +1187,6 @@ func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 
 	if ncolons > 0 {
 		// slice expression
-		if ncolons == 2 && (index[1] == nil || index[2] == nil) {
-			// only i is optional in a[i:j:k]
-			p.error(rbrack, "2nd and 3rd index must be present full slice expression")
-		}
 		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Rbrack: rbrack}
 	}
 

src/pkg/go/parser/short_test.go

@@ -78,12 +78,6 @@ var invalids = []string{
 	`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:k: /* ERROR "expected ']'" */ l] };`,
-	`package p; func f() { var s []int; g(s[::] /* ERROR "index must be present" */) };`,
-	`package p; func f() { var s []int; g(s[i::] /* ERROR "index must be present" */) };`,
-	`package p; func f() { var s []int; g(s[i:j:] /* ERROR "index must be present" */) };`,
-	`package p; func f() { var s []int; g(s[::k] /* ERROR "index must be present" */) };`,
-	`package p; func f() { var s []int; g(s[:j:] /* ERROR "index must be present" */) };`,
-	`package p; func f() { var s []int; g(s[i::k] /* ERROR "index must be present" */) };`,
 }
 
 func TestInvalid(t *testing.T) {