Nested projection normalization results in ambiguity

[compiler-errors]

fn needs_iter(_: impl Iterator<Item = i32>) {}

fn main() {
    needs_iter(IntoIterator::into_iter([0i32; 32]))
}

error[E0284]: type annotations needed: cannot satisfy `<<[i32; 32] as IntoIterator>::IntoIter as Iterator>::Item == i32`
 --> <source>:4:16
  |
4 |     needs_iter(IntoIterator::into_iter([0i32; 32]))
  |     ---------- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `<<[i32; 32] as IntoIterator>::IntoIter as Iterator>::Item == i32`
  |     |
  |     required by a bound introduced by this call
  |
note: required by a bound in `needs_iter`
 --> <source>:1:32
  |
1 | fn needs_iter(_: impl Iterator<Item = i32>) {}
  |                                ^^^^^^^^^^ required by this bound in `needs_iter`

This may be due to us assembling two candidates -- one via the alias bound for IntoIter and one via the normalized self type <[i32; 32] as IntoIterator>::IntoIter which is std::array::IntoIter<i32, 32>...

I'm surprised we're not combining these because they should have the same constraints -- none at all.

[compiler-errors]

Marking this as a "fundamental" issue because we're going to have to hack around it at the solver level, possibly by not preferring alias bounds over impl bounds (or perhaps getting rid of alias bounds altogether)

[lcnr]

I'm surprised we're not combining these because they should have the same constraints -- none at all.

i don't think we combine candidates yet until rust-lang/rust#109724 so i think at least this specific case should be fixed with that?

[compiler-errors]

oh you're right, duh. well glad it's fixed by that.

[compiler-errors]

@lcnr this doesn't seem to be fixed by your pr

[lcnr]

inside of the normalizes-to rhs infer hack:

  Candidate { source: Impl(DefId(2:3663 ~ core[523c]::array::iter::{impl#2})), result: Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [i32] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [] } },
  Candidate { source: AliasBound, result: Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [<[i32; 32] as std::iter::IntoIterator>::Item] }, external_constraints: ExternalConstraints(Interned(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }, PrivateZst)) }, max_universe: U0, variables: [] } }]

normalizing to either i32 or <[i32; 32] as std::iter::IntoIterator>::Item 😅 have a few different ideas here, can chat about them in sync 😁

[compiler-errors]

Moving to rust-lang/trait-system-refactor-initiative#4