Factory methods should have direct access to type parameters

[DartBot]

This issue was originally filed by [email protected]

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class WrapT<T> {
  Type get type => T;
}

class MyClass<T> {
  factory MyClass.works() {
    Type t = new WrapT<T>().type;
    print(t);
  }

  factory MyClass.fails() {
    print(T);
  }
}

main() {
  new MyClass<String>.works();
  new MyClass<String>.fails();
}

Right now, I get: cannot access type parameter 'T' from static function
As the example above shows T is available using wrapper code. So the error should not have to occur for factory methods.

Thanks,
Andreas

[DartBot]

This comment was originally written by @mezoni

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But the "factory" are syntactic sugar over the static methods.
How they can get access to type arguments if Dart language does not support generic methods?

Even if they in Dart available with an operator "new" this does not means that they are constructors.

This is also syntactic sugar over the static methods.

[DartBot]

This comment was originally written by [email protected]

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Thanks for the comment.

As you see above, factory methods have access to the generic type parameters. The first factory constructor actually works. It's just that wrapper code is needed to get around the unnecessary error.

[floitschG]

Removed Type-Defect label.
Added Type-Enhancement, Area-Language, Triaged labels.

[lrhn]

I think this is a bug in the VM.

The spec says
... "Otherwise, the body of q is executed with respect to the bindings that
resulted from the evaluation of the argument list and the type parameters (if
any) of q bound to the actual type arguments V1, . . . , Vl resulting in an object
i. The result of the evaluation of e is i."

This says that the body is executed in a scope where the type parameter is in scope.
Similar code works in dart2js.

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Removed Type-Enhancement, Area-Language labels.
Added Type-Defect, Area-VM labels.

[iposva-google]

Gilad, please verify that the spec interpretation is correct and that the VM should allow direct access to T in the factory. Thanks!

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cc @gbracha.
cc @crelier.

[crelier]

I also think it is a bug.

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Set owner to @crelier.
Added Accepted label.

[crelier]

Fixed at r44339.

Corresponding issues have been opened for dart2js (22738) and analyzer (22739).

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Added Fixed label.