Implicit cast-and-construct from dynamic to extension type?

[srawlins]

The (only?) implicit cast we have left (I think I'm not including coersions...) is "from dynamic to anything". So in this code:

extension type E(int it) {}

void main() {
  dynamic i = 7;
  E j = i;
}

Is this legal? Does i get implicitly cast to E which invokes E's primary constructor? I think this is all expected and fine, but it's not really spelled out in the spec, I think.

CC @eernstg

[eernstg]

Yes, I would expect E j = i; to be accepted without errors. At run time it would check that i is an int (and it wouldn't do anything beyond that).

I think it's fair to say that this treatment is covered by the existing specification of assignability, so there shouldn't be a need to put anything into the extension type specification about it.

However, I would also hope that this initialization could be flagged by a lint saying that a cast to an extension type is a questionable action (cf. dart-lang/sdk#58838).

[lrhn]

extension type E(int it) {}

void main() {
 dynamic i = 7;
 E j = i;
}

Is this legal?

Yes.

Does i get implicitly cast to E

Yes.
After type inference, this should be equivalent to:

 extension type E(int it) {}
 void main() {
   dynamic i = 7;
   E j = i as E; // Implicit downcast from dynamic to context type.
}

There is an implicit downcast from dynamic to the non-top context type, like always.
It doesn't matter that the context type is an extension type.

which invokes E's primary constructor?

No. It just gets cast.

Casts are runtime operations. At runtime, extension types do not exist. They're completely gone, replaced by their representation type.

The code that actually runs will be:

void main() {
  dynamic /*aka Object?*/ i = 7;
  int j = i as int;
}

I think this is all expected and fine, but it's not really spelled out in the spec, I think.

I think it is. It might be implicit, because it is a consequence of the interaction of how extension types work at compile-time and at runtime, and completely standard behavior.

• Implicit downcasts do not care about extension types. They will downcast to any type.
• Casts to extension types happen at runtime where it will be a cast to the representation type.

[srawlins]

Thanks! Filed dart-lang/sdk#59310