co19/LanguageFeatures/Patterns/pattern_assignment_A07_t01

[scheglov]

FAILED: dart2analyzer-none release_arm64 co19/LanguageFeatures/Patterns/pattern_assignment_A07_t01
Expected: Pass
Actual: MissingCompileTimeError

--- Command "dart2analyzer" (took 06.000205s):
DART_CONFIGURATION=ReleaseARM64 sdk/bin/dart --packages=.dart_tool/package_config.json pkg/analyzer_cli/bin/analyzer.dart --use-analysis-driver-memory-byte-store --dart-sdk=/Users/scheglov/Source/Dart/sdk.git/sdk/sdk -Dtest_runner.configuration=custom-configuration-1 --enable-experiment=patterns,records --ignore-unrecognized-flags --packages=/Users/scheglov/Source/Dart/sdk.git/sdk/.dart_tool/package_config.json --format=json /Users/scheglov/Source/Dart/sdk.git/sdk/tests/co19/src/LanguageFeatures/Patterns/pattern_assignment_A07_t01.dart

static error failures:
- Missing expected unspecified error at line 46, column 4, length 1.

- Unexpected error at line 33, column 22, length 2: COMPILE_TIME_ERROR.INVALID_ASSIGNMENT

--- Re-run this test:
python3 tools/test.py -m release -c dart2analyzer -a arm64 co19/LanguageFeatures/Patterns/pattern_assignment_A07_t01

I think code

void f(num a) {
  (a as int) = '';
}

works as

void f(num a) {
  a = '' as int;
}

It will crash at runtime, of course, but statically it is fine.

[sgrekhov]

@eernstg @scheglov I prepared PR fixing this issue. See #1713. But specification reads

The matched value type for a variable or cast pattern is not assignable to the corresponding variable's type.

What should be the right test, expecting a compile-time error in case of cast pattern in a pattern assignment? Could you advice?

[eernstg]

My understanding of the spec sentence is that we should determine whether String (which is the static type of the matched value) is assignable to num (which is the declared type of the corresponding variable). This is not true, and hence I'd expect a compile-time error at the pattern assignment:

void f(num a) {
  (a as int) = ''; // Here.
}

@munificent, is this the intended meaning of that sentence?

[eernstg]

I added a comment on #1713 (which was landed without taking this error into account—so we probably need yet another small fix on LanguageFeatures/Patterns/pattern_assignment_A07_t01.dart).

[munificent]

I think it's just a bug in the proposal. That sentence shouldn't mention cast patterns at all. As @scheglov says, I would expect that assignment to be statically fine but fail at runtime.

I'll fix the proposal.

[eernstg]

Thanks, @munificent! I'll assume this rule about pattern assignments:

The matched value type for a variable pattern is not assignable to the corresponding variable's type.

With this, I think we have a settled approach for the given example:

void f(num a) {
  (a as int) = '';
}

The cast pattern a as int provides int as the matched value type for the variable pattern a, and int is assignable to num, so there's no error.

There are no further type checks on the cast pattern or the pattern assignment as a whole, which means that the example has no compile-time errors.

@sgrekhov, it looks like the test co19/LanguageFeatures/Patterns/pattern_assignment_A07_t01 does not currently contain any cast patterns. Perhaps some tests were renamed along the way? In any case, I think there is some test which contains the example mentioned in this issue, and it should not expect a compile-time error at that point. Do those elements fit together for you, somehow? ;-)

[sgrekhov]

@eernstg co19/LanguageFeatures/Patterns/pattern_assignment_A07_t02 was added. It's a short test containing cast pattern only

main() {
  num d = 3.14;
  Expect.throws(() {
    (d as double) = "one";
  });
}

[eernstg]

OK, thanks! It looks like co19/LanguageFeatures/Patterns/pattern_assignment_A07_t02.dart is already in good shape.

[sgrekhov]

Closing this issue as completed