cmd/compile/internal/syntax: handle missing index like in go/parser

Instead of simply reporting an error but otherwise dropping the
index expression from the parse tree when an index is missing
(as in: x[]), create an index expression with a "bad expression"
as index. This matches the behavior of go/parser and permits the
use of the same test case for both parsers.

(It would be simpler to adjust the go/parser to match the syntax
parser's behavior, but that would break backward-compatibility
of the go/parser.)

Adjust the affected test files.

For #54511.

Change-Id: If7668973794604593e869a24b560da92e100b812
Reviewed-on: https://go-review.googlesource.com/c/go/+/424654
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1111,20 +1111,19 @@ loop:
 		case _Lbrack:
 			p.next()
 
-			if p.tok == _Rbrack {
-				// invalid empty instance, slice or index expression; accept but complain
-				p.syntaxError("expecting operand")
-				p.next()
-				break
-			}
-
 			var i Expr
 			if p.tok != _Colon {
 				var comma bool
+				if p.tok == _Rbrack {
+					// invalid empty instance, slice or index expression; accept but complain
+					p.syntaxError("expecting operand")
+					i = p.badExpr()
+				} else {
 					i, comma = p.typeList()
+				}
 				if comma || p.tok == _Rbrack {
 					p.want(_Rbrack)
-					// x[i,] or x[i, j, ...]
+					// x[], x[i,] or x[i, j, ...]
 					t := new(IndexExpr)
 					t.pos = pos
 					t.X = x

src/cmd/compile/internal/types2/testdata/examples/functions.go

@@ -211,7 +211,7 @@ func _() {
 func h[] /* ERROR empty type parameter list */ () {}
 
 func _() {
-	h[] /* ERROR operand */ ()
+	h[ /* ERROR cannot index */ ] /* ERROR operand */ ()
 }
 
 // Parameterized functions must have a function body.

src/cmd/compile/internal/types2/testdata/fixedbugs/issue39634.go

@@ -83,7 +83,7 @@ var x T25 /* ERROR without instantiation */ .m1
 
 // crash 26
 type T26 = interface{ F26[ /* ERROR interface method must have no type parameters */ Z any]() }
-func F26[Z any]() T26 { return F26 /* ERROR without instantiation */ [] /* ERROR operand */ }
+func F26[Z any]() T26 { return F26[] /* ERROR operand */ }
 
 // crash 27
 func e27[T any]() interface{ x27 /* ERROR not a type */ } { panic(0) }

src/go/types/testdata/fixedbugs/issue39634.go

@@ -83,9 +83,6 @@ var x T25 /* ERROR without instantiation */ .m1
 
 // crash 26
 type T26 = interface{ F26[ /* ERROR interface method must have no type parameters */ Z any]() }
-// The error messages on the line below differ from types2 because for backward
-// compatibility go/parser must produce an IndexExpr with BadExpr index for the
-// expression F26[].
 func F26[Z any]() T26 { return F26[] /* ERROR operand */ }
 
 // crash 27