Downcasting against type parameter failed

[f37yu]

Minimal reproduction:

forall (T) struct wrap {
    T elem;
};

forall (T) void foo (wrap(T) x, T y) {
}



int main() {
    wrap(int) w{10};
    foo (w, 2.0); // error?
}

[f37yu]

To clarify, this particular issue requires a tightly bound parameter T (exact match)
In the call T is forced to be bound to int, so we expect the 2.0 to be (implicitly) downcast to int, but the unify algorithm does not consider downcast as a possibility and only attempts to unify T=int with double (by arithmetic hierarchy) Since T cannot be changed to double it fails.

Actually, the box type is not necessary. We can achieve the tightly bound T by using a pointer instead:

forall (T) void foo (T * x, T y);
int z;
foo (&z, 2.0); // also fails