Default type argument is not checked

[simonthum]

This passes the type checker:

shared interface TraitB<Representation  = Integer> given Representation satisfies TraitB<Representation> {
    shared formal Representation modfiyB();
}

But you cannot do much without assigning some legal type to Representation.

[lucaswerkmeister]

Note that this is a typechecker bug, not a spec oversight: The spec clearly states that

The default type argument must satisfy the constraints on the type parameter.

[simonthum]

@lucaswerkmeister I am unsure if I understand you correctly.

My assumption is that it is not currently checked, because it passes. It plainly does not make nay sense not to check it. So it would be an oversight or a bug - I just assumed the spec contains this simple corollary.

Then, as far as I know, this repo contains the typechecker, so this seems the right place. Anything wrong?

[lucaswerkmeister]

No, everything is correct. I just wanted to clarify that is a pure bug, and not something that was forgotten in the textual specification.

[simonthum]

Sure, thanks for making up for my brevity!

[FroMage]

Looks like a bug, thanks!

[lucaswerkmeister]

Some more info:

class K<T=Integer>()
        given T satisfies String {
}

void run() {
    K k1 = nothing; // well-typed, and the IDE says it is indeed a K<Integer>
    K<Integer> k2 = nothing; // error: Integer not assignable to upper bound String
    K k3 = K(); // error: K<Nothing> not assignable to K<Integer> – this error vanishes if you declare T as out
}

Note especially k3 – it seems the typechecker is constructing the intersection of the default type argument and the type parameter’s constraint when instantiating the class.

[gavinking]

Fixed by @928061f.