Trait implementation solving fails for a closure

[akatsukilelouch]

I tried this code:

struct Foo<A, B>(A, B);

#[allow(non_camel_case_types, dead_code)]
trait Fn_require_outlives_in_method<A, B> {
    // a for<'a> alternative that is type checked
    // at the call site
    fn call<'a>(self, value: &'a A) -> &'a B;
}

// impl the trait for FnOnce(&A) -> &B
impl<A, B, F: for<'a> FnOnce(&'a A) -> &'a B /* where A: 'a, B: 'a (trivial bounds) */>
Fn_require_outlives_in_method<A, B> for F {
    #[inline(always)]
    fn call<'a>(self, value: &'a A) -> &'a B {
        self(value)
    }
}

impl<A, B> Foo<A, B> {
    fn new(_value: A, _callback: impl Fn_require_outlives_in_method<A, B>) -> Self {
        todo!()
    }
}

fn check_fn_once_impl_universal_over_lifetime_a(_: impl for<'a> FnOnce(&'a i32) -> &'a i32) {}

fn main() {
    Foo::new(0, |e| e);
}

I expected to see this happen: compiles

Instead, this happened:

error: implementation of `FnOnce` is not general enough
  --> src/main.rs:28:5
   |
28 |     Foo::new(0, |e| e);
   |     ^^^^^^^^ implementation of `FnOnce` is not general enough
   |
   = note: closure with signature `fn(&'2 i32) -> &i32` must implement `FnOnce<(&'1 i32,)>`, for any lifetime `'1`...
   = note: ...but it actually implements `FnOnce<(&'2 i32,)>`, for some specific lifetime `'2`

Other examples with comments:

fn check_fn_once_impl_universal_over_lifetime_a(_: impl for<'a> FnOnce(&'a i32) -> &'a i32) {}

fn main() {
    fn test(e: &i32) -> &i32 { e }

    // Foo::new(0, |e| e); // fails to compile (not general enough)
    // Foo::<i32, i32>::new(0, |e| e); // fails to compile (not general enough)

    check_fn_once_impl_universal_over_lifetime_a(|e| e); // ok

    Foo::new(0, for<'a> |e: &'a i32| -> &'a i32 { e }); // ok (experimental)
    Foo::new(0, test); // ok
}

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (75530e9f7 2025-03-18)
binary: rustc
commit-hash: 75530e9f72a1990ed2305e16fd51d02f47048f12
commit-date: 2025-03-18
host: aarch64-apple-darwin
release: 1.87.0-nightly
LLVM version: 20.1.0

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