Generic fn cannot be converted to for<'a> Fn

[theemathas]

This code causes a compile error, but I think that it should compile without problems.

use std::marker::PhantomFn;

struct Foo(Bar);
struct Bar;

trait BorrowFn<'a>: PhantomFn<&'a Self, ()> {}
impl<'a, T: 'a, F: Fn(&'a Foo) -> T> BorrowFn<'a> for F {}

fn something<F>(_: F) where for<'a> F: BorrowFn<'a> {}

fn foo_as_bar<'a>(x: &'a Foo) -> &'a Bar { &x.0 }

fn main() {
    something(foo_as_bar);
}

playpen

The compile errors:

<anon>:14:5: 14:14 error: the requirement `for<'a> _ : 'a` is not satisfied [E0280]
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~
<anon>:14:5: 14:14 error: type mismatch resolving `for<'a> <fn(&'a Foo) -> &'a Bar {foo_as_bar} as core::ops::Fn<(&'a Foo,)>>::Output == _`:
 expected bound lifetime parameter 'a,
    found concrete lifetime [E0271]
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~
<anon>:14:5: 14:14 note: required by `something`
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~

cc @reem This is from the code I showed to you on IRC.

[theemathas]

Changing the lifetime names to ease understanding

use std::marker::PhantomFn;

struct Foo(Bar);
struct Bar;

trait BorrowFn<'a>: PhantomFn<&'a Self, ()> {}
impl<'b, T: 'b, F: Fn(&'b Foo) -> T> BorrowFn<'b> for F {}

fn something<F>(_: F) where for<'c> F: BorrowFn<'c> {}

fn foo_as_bar<'d>(x: &'d Foo) -> &'d Bar { &x.0 }

fn main() {
    something(foo_as_bar);
}

playpen

The errors:

<anon>:14:5: 14:14 error: the requirement `for<'c> _ : 'c` is not satisfied [E0280]
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~
<anon>:14:5: 14:14 error: type mismatch resolving `for<'c> <fn(&'d Foo) -> &'d Bar {foo_as_bar} as core::ops::Fn<(&'c Foo,)>>::Output == _`:
 expected bound lifetime parameter 'c,
    found concrete lifetime [E0271]
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~
<anon>:14:5: 14:14 note: required by `something`
<anon>:14     something(foo_as_bar);
              ^~~~~~~~~

Note that for<'c> <fn(&'d Foo) -> ... as ...> doesn't really make sense. I don't think that 'd should appear there.

[Aatch]

The 'd lifetimes are from foo_as_bar.

[theemathas]

Possibly related to #22340

[apasel422]

This is still an issue after removing the outdated PhantomFn bounds.

[steveklabnik]

@apasel422 do you remember how to do so? getting a compileable example in this ticket would be helpful

[apasel422]

@steveklabnik

I think the current version of this code is:

struct Foo(Bar);
struct Bar;

trait BorrowFn<'a> {}
impl<'b, T: 'b, F: Fn(&'b Foo) -> T> BorrowFn<'b> for F {}

fn something<F>(_: F) where for<'c> F: BorrowFn<'c> {}

fn foo_as_bar<'d>(x: &'d Foo) -> &'d Bar { &x.0 }

fn main() {
    something(foo_as_bar);
}

[Mark-Simulacrum]

Closing in favor of #41078.