Checking that `main`'s output type implements `Termination` doesn't normalize associated types

[SkiFire13]

I tried this code:

#![feature(termination_trait_lib)]

trait Same {
    type Output;
}
impl<T> Same for T {
    type Output = T;
}
type Unit = <() as Same>::Output;

fn test() {
    fn foo() -> Result<Unit, std::io::Error> { todo!() }

    fn assert_impl_termination<T: std::process::Termination>(_: fn() -> T) {}
    // Compiles
    assert_impl_termination(foo);
}

// Doesn't compile
fn main() -> Result<Unit, std::io::Error> {
    todo!()
}

Playground link

I expected to see both test and main compile, instead only test compiles, while trying to compile main gives this error:

error[E0277]: `main` has invalid return type `Result<<() as Same>::Output, std::io::Error>`
  --> src/main.rs:20:14
   |
20 | fn main() -> Result<Unit, std::io::Error> {
   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` can only return types that implement `Termination`
   |
   = help: consider using `()`, or a `Result`
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
   |
20 | fn main() -> Result<Unit, std::io::Error> where Result<<() as Same>::Output, std::io::Error>: Termination {
   |                                           +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0277`.

Note that the feature attribute is only needed for the bound in assert_impl_termination. The issue with main happens both in stable and nightly, with or without the feature.

Meta

rustc --version --verbose:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-pc-windows-msvc
release: 1.54.0
LLVM version: 12.0.1

rustc 1.56.0-nightly (29ef6cf16 2021-08-31)
binary: rustc
commit-hash: 29ef6cf1637aa8317f8911f93f14e18d404c1b0e
commit-date: 2021-08-31
host: x86_64-pc-windows-msvc
release: 1.56.0-nightly
LLVM version: 13.0.0

[ThePuzzlemaker]

I am going to attempt to work on this. I've already got a solution that seems to work, but it's probably not the best way.

@rustbot claim