`Sized` check inconsistently checks/ignores lifetimes

This issue is similar to #156694.

I'm not sure if this is a bug.

pub trait Trait {
    type Assoc<'a>: ?Sized;
}

// This compiles
pub fn works<'a, 'b, T: Trait<Assoc<'a>: Sized>>(_x: T::Assoc<'b>) {
}

// This errors
pub fn fails<'a, 'b, T: Trait<Assoc<'a>: Sized>>(x: T::Assoc<'b>) {
    let _x = x;
}

In the above code, I've specified that T::Assoc<'a>: Sized. However, the code tries to use T::Assoc<'b>: Sized. A function argument needs to be Sized, which makes it a bit weird that works compiles. However, once we've accepted the argument, moving the variable somehow requires Sized in a way such that the compiler checks lifetimes, making fails compile error. This is strange.

pub trait Trait {
    type Assoc<'a>: ?Sized;
}

// This compiles
pub fn works<'a, 'b, T: Trait<Assoc<'a>: Sized>>() {
    let f = || -> T::Assoc<'b> { todo!() };
    let _x = f();
}

// This errors
pub fn fails<'a, 'b, T: Trait<Assoc<'a>: Sized>>() {
    let f = || -> T::Assoc<'b> { todo!() };
    let x = f();
    let _y = x;
}

In the above code, I've again specified that T::Assoc<'a>: Sized, but required T::Assoc<'b>: Sized. Normally, declaring a closure to return a type requires that type to implement Sized, but that check seems to ignore lifetimes. Even calling the closure and putting the result into a variable is allowed. However, for some reason, moving that result into a second variable causes the compiler to check the lifetimes. This is strange.

cc @lcnr

Meta

Reproducible on the playground with version 1.97.0-nightly (2026-05-19 e50aa6fba4e63ab34c72)

Reproduce both pairs of works and fails on the Rust playground using nightly 1.97.0 (2026-05-19 e50aa6fba4e63ab34c72). Compare when the compiler checks Sized for the associated type across the two lifetimes and across the closure and variable moves. Done means the intended lifetime behavior is established and the inconsistent diagnostics or acceptance is resolved.