todo! and unimplemented! does not work with impl Trait return types

[neckbosov]

This code

trait SomeTrait {
    fn some_func();
}

fn todo_impl_trait() -> impl SomeTrait { todo!() }

does not compile because
the trait SomeTrait is not implemented for ()
But such code

trait SomeTrait {
    fn some_func();
}

fn todo_impl_trait<T: SomeTrait>() -> T { todo!() }

compiles correctly. Can this problem be resolved to use both todo!() and impl Trait?

[jonas-schievink]

This is expected behavior since todo!() is not constrained to any type, so it is inferred to (). In the second case it is constrained to T, which is provided by the caller.

[robinmoussu]

Given that 4 bugs have been opened in such a short period of time for the same reason, I don't think that a wontfix, works as intented is the right approach.

At minimum an alternative should be provided in the help message from the compiler, but I personnaly think that todo!(), unimplemented!() and the like should typecheck to ! (never), that itsel should typecheck to any impl Trait.

[jyn514]

todo!(), unimplemented!() and the like should typecheck to ! (never), that itsel should typecheck to any impl Trait.

rust/src/librustc_interface/util.rs

Lines 587 to 594 in 567ad74

	// FIXME: Currently the `everybody_loops` transformation is not applied to:
	// * `const fn`, due to issue #43636 that `loop` is not supported for const evaluation. We are
	// waiting for miri to fix that.
	// * `impl Trait`, due to issue #43869 that functions returning impl Trait cannot be diverging.
	// Solving this may require `!` to implement every trait, which relies on the an even more
	// ambitious form of the closed RFC #1637. See also [#34511].
	//
	// [#34511]: https://github.com/rust-lang/rust/issues/34511#issuecomment-322340401

[jonas-schievink]

Reopening this to have a canonical issue to direct people to. This is still working as expected for the moment though.

[jonas-schievink]

Actually, let's classify this as a diagnostics enhancement