Type inference can't resolve through Result::map_err

[smmalis37]

I would expect all 4 of these functions to compile, but the second one apparently doesn't have enough type information, despite it looking logically equivalent to the first (to me anyways).

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=1d3fa8810715f720dd3f6c09e8db534f

[Emerentius]

? doesn't desugar to an .into() call in the error case, but to a from call. If you use from explicitly, it does compile:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b780bef6975cee61c307dc9ec0a73584

I think this is expected behavior, because a method call requires that the receiver's type is known in advance.

[smmalis37]

I had thought that the type inference would be able to essentially follow this path of logic:

1. The return type of the function is Result<u32, Box<dyn Error>>.
2. map_err does not change the type on the Ok side of the Result (u32).
3. Therefore the return type of parse must be Result<u32, _>.
4. Therefore the generic parameter on parse must be u32.
5. Therefore the type of x must be ParseIntError.

Should it be able to? IDK