Error converting from i32 to f64 when type parameter implements From<f64>

[djc]

I tried this code:

fn foo<T>(x: T, y: i32) -> f64 where f64: From<T> {
    f64::from(y)
}

I expected to see this happen: should compile and do the right thing.

Instead, this happened:

error[E0308]: mismatched types
 --> src/lib.rs:2:15
  |
1 | fn foo<T>(x: T, y: i32) -> f64 where f64: From<T> {
  |        - this type parameter
2 |     f64::from(y)
  |               ^ expected type parameter `T`, found `i32`
  |
  = note: expected type parameter `T`
                       found type `i32`
  = help: type parameters must be constrained to match other types
  = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters

On the other hand, this code compiles fine:

fn foo<T>(_: T, y: i32) -> f64 {
    f64::from(y)
}

Meta

I was surprised about this, so after running this with 1.44.0 and 1.44.1 I checked older versions, but 1.39.0, 1.31.0 and 1.15.0 all give the same error. Seems likely that there is an issue about this already, but no clue how to look for it.

[tesuji]

I don't know if we can bound on primitive types like f64. But if you bound T by Into trait, this should compile:

fn foo<T>(x: T, y: i32) -> f64 where T: Into<f64>;

[lcnr]

Might be related to #34979 (comment)

I think this is caused by winnowing somehow incorrectly discarding a relevant predicate.

cc @eddyb @nikomatsakis for good measure

[nikomatsakis]

Yeah, I agree with @lcnr's diagnosis. This is an example of a known limitation with the current trait solver, sort of a sub-issue of #41756 or some other similar issues. I'm not sure that we have any plans to fix this apart from transitioning over to chalk.