unimplemented macro doesnt compile with RPIT

[95th]

I believe the following should compile but doesn't.

trait Foo {}

fn bar() -> impl Foo {
    unimplemented!()
}

I get the following error:

 --> src/main.rs:7:13
  |
7 | fn bar() -> impl Foo {
  |             ^^^^^^^^ the trait `Foo` is not implemented for `()`
  |
  = note: the return type of a function must have a statically known size

However, this compiles:

fn bar<X: Foo>() -> X {
    unimplemented!()
}

Could anybody confirm if this is a bug or I am missing something very obvious?

[jonas-schievink]

This isn't really a bug, at least. The type impl Foo stands for needs to be inferred somehow, and right now the compiler defaults to () in this situation. With #![feature(never_type)] it would default to !, with the same result (right now).

In the second example, the type is just a type parameter specified by the caller, so no issue occurs.

[95th]

@jonas-schievink The compiler shouldn't default to () for impl Foo. So doesn't that make it a bug?
Also, with never type !, the trait bound !: Foo is not satisfied.

This would mean that you can't leave functions having impl trait return types unimplemented.

[jonas-schievink]

What type should it choose, then? I don't think this is an easy question to answer in all cases.

Diverging control flow (which is what you get after a panic) can generally be of any type, since the code is unreachable. Rustc falls back to assigning () if there is no known, concrete type to assign (this has been the case since before 1.0, I believe). This doesn't really have much to do with impl Trait, specifically.

That said, it would be nice to get this example working somehow, since unimplemented!() would be quite useful.

[memoryruins]

Duplicate of #55022 ?

[jonas-schievink]

Yup, closing