Defaulted const generic parameters no longer have the const arg's type checked

[BoxyUwU]

Code

I tried this code:

struct Foo<const N: u32, const M: u64 = N>;

I expected to see this happen: It fail to compile like it uised to

Instead, this happened: it compile :>

Probably regressed in #125915 where we started lowering const arguments to ConstArgKind::Path instead of ConstArgKind::Anon so there is no longer implicitly an anon const with an expected type of u64 with an expr typed as u32.

We could probably special case handling of types of defaulted const generics in wfcheck to be done even when generics are involved. Or just accept this as working code which is consistent with things like struct Foo<T, U = <T as Iterator>::Item>(T, U); 🤷‍♀ would need a types fcp for that tho

I don't believe this to be a soundness bug

[BoxyUwU]

P-low because it's not going to regress anyone, and its not unsound, and is potentially just a bug fix and is in line with how defaulted generic parameters work in general. cc @rust-lang/types to see if anyone feels strongly about not just accepting this code

[compiler-errors]

Or just accept this as working code which is consistent with things like struct Foo<T, U = <T as Iterator>::Item>(T, U);

Yeah, we already don't check the well-formedness of default types, so I think not checking the well-formedness of default consts is inevitable here.

[lcnr]

I feel somewhat strongly about it.

We currently have the approach of: if the default is fully concrete, check its bounds. I would like to extend this to if the type of the default is fully concrete, check ConstArgHasType for it

[compiler-errors]

I would like to extend this to if the type of the default is fully concrete, check ConstArgHasType for it

We already do that, I think?

struct Foo<const N: u32, const M: u64 = "">;

error[E0308]: mismatched types
 --> src/lib.rs:1:41
  |
1 | struct Foo<const N: u32, const M: u64 = "">;
  |                                         ^^ expected `u64`, found `&str`

For more information about this error, try `rustc --explain E0308`.

[lcnr]

No, we check ConstArgHasType if the const itself is concrete. This issue is that for generic constants with concrete types, we don't check ConstArgHasType for that concrete type