inherent assoc tys resolve with subtyping not equality

[BoxyUwU]

I tried this code:

#![feature(inherent_associated_types)]

struct Foo<T>(T);

impl Foo<fn(&'static ())> {
    type Assoc = u32;
}

trait Other {}
impl Other for u32 {}

fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}

I expected to see this happen: It not to compile as Foo<for<'a> fn(&'a ())> has no inherent impl defined on it

Instead, this happened: it compiled

Meta

playground's nightly:

Build using the Nightly version: 1.70.0-nightly
(2023-03-29 17c11672167827b0dd92)

[fmease]

Fixed in #109410, although the error message is not as nice since we don't try to prettify it (yet). That can be done at some point later though, IMO. The trait analog results in:

error: implementation of `Mid` is not general enough
  --> src/main.rs:17:11
   |
17 | fn bar(_: <Foo<for<'a> fn(&'a ())> as Mid>::Assoc) {}
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Mid` is not general enough
   |
   = note: `Foo<for<'a> fn(&'a ())>` must implement `Mid`
   = note: ...but `Mid` is actually implemented for the type `Foo<fn(&'static ())>`

while the IAT one leads to the following error(s) in #109410:

error[E0308]: mismatched types
  --> tests/ui/associated-inherent-types/issue-109789.rs:17:1
   |
17 | fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected struct `Foo<fn(&'static ())>`
              found struct `Foo<for<'a> fn(&'a ())>`

error[E0308]: mismatched types
  --> tests/ui/associated-inherent-types/issue-109789.rs:17:11
   |
17 | fn bar(_: Foo<for<'a> fn(&'a ())>::Assoc) {}
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected struct `Foo<fn(&'static ())>`
              found struct `Foo<for<'a> fn(&'a ())>`

I should definitely fix the duplication of the diagnostic (albeit harmless).
As mentioned before, we spruce up the region error for traits. This is done in try_report_trait_placeholder_mismatch. If I disable this logic (by modifying it to always return None), the trait analog yields almost the same error message as #109410:

error[E0308]: mismatched types
  --> analog.rs:17:11
   |
17 | fn bar(_: <Foo<for<'a> fn(&'a ())> as Mid>::Assoc) {}
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected trait `<Foo<for<'a> fn(&'a ())> as Mid>`
              found trait `<Foo<fn(&'static ())> as Mid>`

Edit: Well, on a closer look, expected/found is swapped, hmm.

@rustbot claim

[fmease]

Regression test added.