Files
sdk/tests/language/patterns/identifier_when_not_test.dart
T
Paul Berry 637dd76c7f Patterns parsing: fix ambiguity resolution for when and as.
This change fixes parsing of case clauses such as:

    case foo when !flag:

Constructions like this require some lookahead in order to parse
correctly, because the token `when` is valid both as an identifier and
as a part of the grammar for a case clause. Therefore, at the time
`foo` is encountered, the parser must decide whether it is looking at
a variable pattern (`foo when`, where `when` is the name of the
variable) or an identifier pattern (`foo`, where `when` begins the
case's guard clause). Previous to this fix, the algorithm for
disambiguating these two choices was as follows:

- If the token sequence starting at `foo` looked like a type, and the
  token that follows was an identifier, the parser assumed it was
  looking at a variable pattern with a type; otherwise it assumed it
  was looking at an identifier pattern.

- EXCEPT that if the token that followed the supposed type was `when`
  or `as` (both of which are valid identifiers), then it probed
  further:

- If the token that followed `when` or `as` was a token that could
  legitimately follow a pattern, then it assumed that it was looking
  at a variable pattern with a type. (The tokens that could
  legitimately follow a pattern are `,`, `:`, `||`, `&&`, `)`, `}`,
  `]`, `as`, `when`, `?`, `!`).

- Otherwise it assumed that it was looking at an identifier pattern.

This didn't fully disambiguate, because the third bullet didn't
account for the fact that the tokens `as`, `when`, and `!` could
_either_ legitimately follow a pattern _or_ legitimately begin an
expression (or, in the case of `when`, a type), therefore constructs
like the following were incorrectly parsed:

- `case foo when as:` (where `as` is a local boolean variable)
- `case foo when when:` (where `when` is a local boolean variable)
- `case foo when !flag:` (where `flag` is a local boolean variable)
- `case foo as when:` (where `when` is the name of a type)

The solution is to simplify the disambiguation logic so that if if the
token that follows the supposed type is `when` or `as`, then the
parser assumes that it's looking at an identifier pattern, _not_ a
typed variable pattern.

The consequence of this is that the above four constructions are
parsed correctly; however it is no longer possible for a typed
variable pattern to name a variable `when` or `as`.

For consistency we would like to prohibit _any_ variable pattern from
naming a variable `when` or `as`, however to keep this change as small
as possible (and reduce the risk involved in a possible cherry-pick)
that will be postponed until a later CL.

Fixes #52199.

Bug: https://github.com/dart-lang/sdk/issues/52199
Change-Id: Ibab9b92f01e3e4020d7d64f1ff000a9b964a4564
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/299400
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
2023-04-28 13:22:28 +00:00

212 lines
4.4 KiB
Dart

// Copyright (c) 2023, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
// SharedOptions=--enable-experiment=patterns
/// Tests that guarded patterns of the form `identifier when !condition` can be
/// properly parsed. See https://github.com/dart-lang/sdk/issues/52199.
import 'package:expect/expect.dart';
import 'identifier_when_not_test.dart' as self;
main() {
{
// Switch statement: const in scope
switch (1) {
case one when !false:
break;
default:
Expect.fail('Should have matched');
}
}
{
// Switch statement: prefixed identifier
switch (1) {
case self.one when !false:
break;
default:
Expect.fail('Should have matched');
}
}
{
// Switch statement: static const
switch (1) {
case Values.one when !false:
break;
default:
Expect.fail('Should have matched');
}
}
{
// Switch statement: prefixed static const
switch (1) {
case self.Values.one when !false:
break;
default:
Expect.fail('Should have matched');
}
}
{
// Switch expression: const in scope
(switch (1) { one when !false => 0, _ => throw 'Should have matched' });
}
{
// Switch expression: prefixed identifier
(switch (1) {
self.one when !false => 0,
_ => throw 'Should have matched'
});
}
{
// Switch expression: static const
(switch (1) {
Values.one when !false => 0,
_ => throw 'Should have matched'
});
}
{
// Switch expression: prefixed static const
(switch (1) {
self.Values.one when !false => 0,
_ => throw 'Should have matched'
});
}
{
// If-case statement: const in scope
if (1 case one when !false) {
} else {
Expect.fail('Should have matched');
}
}
{
// If-case statement: prefixed identifier
if (1 case self.one when !false) {
} else {
Expect.fail('Should have matched');
}
}
{
// If-case statement: static const
if (1 case Values.one when !false) {
} else {
Expect.fail('Should have matched');
}
}
{
// If-case statement: prefixed static const
if (1 case self.Values.one when !false) {
} else {
Expect.fail('Should have matched');
}
}
{
// If-case in list: const in scope
Expect.listEquals(
[''], [if (1 case one when !false) '' else 'Should have matched']);
}
{
// If-case in list: prefixed identifier
Expect.listEquals(
[''], [if (1 case self.one when !false) '' else 'Should have matched']);
}
{
// If-case in list: static const
Expect.listEquals([''],
[if (1 case Values.one when !false) '' else 'Should have matched']);
}
{
// If-case in list: prefixed static const
Expect.listEquals([
''
], [
if (1 case self.Values.one when !false) '' else 'Should have matched'
]);
}
{
// If-case in map: const in scope
Expect.mapEquals({'': ''},
{if (1 case one when !false) '': '' else '': 'Should have matched'});
}
{
// If-case in map: prefixed identifier
Expect.mapEquals({
'': ''
}, {
if (1 case self.one when !false) '': '' else '': 'Should have matched'
});
}
{
// If-case in map: static const
Expect.mapEquals({
'': ''
}, {
if (1 case Values.one when !false) '': '' else '': 'Should have matched'
});
}
{
// If-case in map: prefixed static const
Expect.mapEquals({
'': ''
}, {
if (1 case self.Values.one when !false)
'': ''
else
'': 'Should have matched'
});
}
{
// If-case in set: const in scope
Expect.setEquals(
{''}, {if (1 case one when !false) '' else 'Should have matched'});
}
{
// If-case in set: prefixed identifier
Expect.setEquals(
{''}, {if (1 case self.one when !false) '' else 'Should have matched'});
}
{
// If-case in set: static const
Expect.setEquals({''},
{if (1 case Values.one when !false) '' else 'Should have matched'});
}
{
// If-case in set: prefixed static const
Expect.setEquals({
''
}, {
if (1 case self.Values.one when !false) '' else 'Should have matched'
});
}
}
const one = 1;
class Values {
static const one = 1;
}