d9aa287672
dont provide fwd declared params to cg defaults Fixes #83938 ```rust #![feature(const_evaluatable_checked, const_generics, const_generics_defaults)] #![allow(incomplete_features)] pub struct Bar<const N: usize, const M: usize = { N + 1 }>; pub fn foo<const N1: usize>() -> Bar<N1> { loop {} } fn main() {} ``` This PR makes this code no longer ICE, it was ICE'ing previously because when building substs for `Bar<N1>` we would subst the anon ct: `ConstKind::Unevaluated({N + 1}, substs: [N, M])` with substs of `[N1]`. the anon const has forward declared params supplied though so we end up trying to substitute the provided `M` param which causes the ICE. This PR doesn't handle the predicates of the const so ```rust trait Foo<const N: usize> { const Assoc: usize; } pub struct Bar<const N: usize = { <()>::Assoc }> where (): Foo<N>; ``` Resolves to `<() as Foo<N>>::Assoc` which can allow for using fwd declared params indirectly. ```rust trait Foo<const N: usize> {} struct Bar<const N: usize = { 2 + 3 }> where (): Foo<N>; ``` This code also ICEs under this PR because instantiating the default's predicates causes an ICE as predicates_of contains predicates with fwd declared params PR was briefly discussed [in this zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/evil.20preds.20in.20param.20env.20.2386580) |
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