Closures needlessly capture all generic parameters from parent item in TAIT

I tried this code:

#![feature(type_alias_impl_trait)]

type Tait = impl Sized + 'static;

fn capture<T>() -> Tait {
    || 0u8
}

I expected to see this happen:

Successful compilation, since the generic parameter T is not used in the returned closure.

Instead, this happened:

   Compiling playground v0.0.1 (/playground)
error: type parameter `T` is part of concrete type but not used in parameter list for the `impl Trait` type alias
 --> src/lib.rs:6:5
  |
6 |     || 0u8
  |     ^^^^^^

This error also happens when returning async blocks:

// same error
fn capture_async<T>() -> Tait {
    async {
        0u8
    }
}

Also, this appears to only affect generic type parameters, as lifetimes aren't captured if the closure doesn't use them:

// compiles
fn capture_lt<'a>(_: &'a ()) -> Tait {
    || 0u8
}

Meta

Version:

rustc 1.76.0-nightly (2023-11-19 9a66e4471f71283fd54d)

Start with the reproducer in src/lib.rs using type_alias_impl_trait, then compare the closure and async-block cases with the lifetime example. Done means the generic parameter is not captured when unused and the examples compile successfully, with regression coverage for the reported behavior.