asA generates code which prevents an expression from being an arrow function

[srawlins]

I added this test case to test/specs/code/expression_test.dart around line 470:

test('should emit an explicit cast in an arrow function', () {
    expect(
      refer('callback').call([
        Method((b) => b..body = refer('foo').asA(refer('String')).code).closure
      ]),
      equalsDart('callback(() => ( foo as String ))'),
    );
  });

The test fails because the output is actually callback(() { ( foo as String )... which

a. does not have the expected =>, and
b. does not include a semi-colon, so the output is not even parseable Dart.

[natebosch]

To get a => method you want to use b..lambda = true.

  test('should emit an explicit cast in an arrow function', () {
    expect(
      refer('callback').call([
        Method((b) => b
          ..lambda = true
          ..body = refer('foo').asA(refer('String')).code).closure
      ]),
      equalsDart('callback(() => ( foo as String ))'),
    );
  });

[srawlins]

Ah thanks, that works.

Is the bug still valid in that the snippet above should at least generate parseable code?

[srawlins]

To be more precise, what seemed weird was that I can normally just chain some expressions together and say b.body = that expression, but as soon as you attach .asA, its broken:

// OK
final returnNoSuchMethod = refer('super')
    .property('noSuchMethod')
    .call(noSuchMethodArgs);
builder.body = returnNoSuchMethod.code;

// UNPARSEABLE
final returnNoSuchMethod = refer('super')
    .property('noSuchMethod')
    .call(noSuchMethodArgs)
    .asA(_typeReference(method.returnType));
builder.body = returnNoSuchMethod.code;

[natebosch]

Ah I see what the problem is.

We do attempt to allow you to leave the lambda property inferred if you have a non-statement single expression (that's expected to be the only way to end up with a ToCodeExpression). https://github.com/dart-lang/code_builder/blob/7203e376415de0eb6a5370f07e79c8a8d78d35d5/lib/src/emitter.dart#L86-L87

The problem is that there is a way to end up with a Block instead because I hacked in parenthesis using that instead of implementing a parenthesizedExpression properly. https://github.com/dart-lang/code_builder/blob/7203e376415de0eb6a5370f07e79c8a8d78d35d5/lib/src/specs/expression.dart#L58-L66

I don't know how difficult it will be to fix this. We may need a breaking change to add a new expression type.

cc @matanlurey

[matanlurey]

I'd be OK with a breaking change