The "type_alias_impl_trait" feature has problems handling lifetime parameters.

[steffahn]

I was starting with this function (which does compile on nightly):

#![feature(unboxed_closures)]
#![feature(type_alias_impl_trait)]
fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
    -> impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a
{
    move |a| move |b| f(a,b)
}

and wanted to give a name to its return type.
The straightforward approach seems to be (please correct me if the "right" way to do this is different):

#![feature(unboxed_closures)]
#![feature(type_alias_impl_trait)]
type Curried<'a, A: 'a, B, C, F: Fn(A, B) -> C>
 = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;

fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
    -> Curried<'a, A, B, C, F>
{
    move |a| move |b| f(a,b)
}

But the compiler is currently unhappy with the lifetimes. I'm getting the following.

error: cannot infer an appropriate lifetime
  --> src/main.rs:11:5
   |
6  |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
   |                           ------------------------ this return type evaluates to the `'static` lifetime...
...
11 |     move |a| move |b| f(a,b)
   |     ^^^^^^^^^^^^^^^^^^^^^^^^ ...but this borrow...
   |
note: ...can't outlive the lifetime `'a` as defined on the function body at 8:10
  --> src/main.rs:8:10
   |
8  | fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
   |          ^^
help: you can add a bound to the return type to make it last less than `'static` and match the lifetime `'a` as defined on the function body at 8:10
   |
9  |     -> Curried<'a, A, B, C, F> + '_
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: cannot infer an appropriate lifetime
  --> src/main.rs:11:14
   |
6  |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
   |                           ------------------------ this return type evaluates to the `'static` lifetime...
...
11 |     move |a| move |b| f(a,b)
   |              ^^^^^^^^^^^^^^^ ...but this borrow...
   |
note: ...can't outlive the lifetime `'a` as defined on the function body at 8:10
  --> src/main.rs:8:10
   |
8  | fn curry<'a, A: 'a, B, C, F: Fn(A, B) -> C> (f: &'a F)
   |          ^^
help: you can add a bound to the return type to make it last less than `'static` and match the lifetime `'a` as defined on the function body at 8:10
   |
9  |     -> Curried<'a, A, B, C, F> + '_
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

It works without the lifetimes (i.e. after removing parametrization over 'a and replacing all occurrences of 'a with 'static), like this:

#![feature(unboxed_closures)]
#![feature(type_alias_impl_trait)]
type Curried<A: 'static, B, C, F: Fn(A, B) -> C>
 = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'static> + 'static;

fn curry<A: 'static, B, C, F: Fn(A, B) -> C> (f: &'static F)
    -> Curried<A, B, C, F>
{
    move |a| move |b| f(a,b)
}

use std::ops::Add;
fn main() {
    let x = curry(&i32::add)(1)(2);
    println!("{}", x); // prints "3"
}

Meta

Current behavior only since #67844 was fixed (after nightly-2020-02-14). Before that, this code triggered an ICE.

[Aaron1011]

When I wrote #69008, I assumed that the three types of generic substs (types, lifetimes, and consts) were all handled in the same way for opaque types. However, this is not the case: the handling of lifetimes is significantly more complicated than that of types or consts.

I still think that using the 'parent' opaque type as the 'parent' generics is the best approach. However, it's going to require several changes to how we process lifetimes. In particular, we'll need to ensure that simply using a lifetime (e.g. impl FnOnce(B) -> C + 'a) does not create a corresponding generic parameter when we're outside of a return-position-impl-trait context.

[jackh726]

Errors on master give correct suggestion

error[E0309]: the parameter type `F` may not live long enough
 --> src/main.rs:4:4
  |
4 |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: consider adding an explicit lifetime bound `F: 'a`...
  = note: ...so that the type `[closure@src/main.rs:9:5: 9:29]` will meet its required lifetime bounds

error[E0309]: the parameter type `F` may not live long enough
 --> src/main.rs:4:27
  |
4 |  = impl Fn<(A,), Output = impl FnOnce(B) -> C + 'a> + 'a;
  |                           ^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: consider adding an explicit lifetime bound `F: 'a`...
  = note: ...so that the type `[closure@src/main.rs:9:14: 9:29]` will meet its required lifetime bounds

error: aborting due to 2 previous errors

Change Curried to be type Curried<'a, A: 'a, B, C, F: 'a + Fn(A, B) -> C> = ... compiles successfully.

This just needs to be added as a test.

[steffahn]

Seems like this works (with the additional F: 'a) since #72080 (nightly-2020-06-16).

[steffahn]

Actually it used to work without the F: 'a for a while. From 2020-06-16 up to 2020-10-06 after which #73905 was merged. I feel like that it working without the 'a bound on F might potentially have been unsound, so I guess my original code stands corrected.

[nikomatsakis]

For min_type_alias_impl_trait, this is #86731