Trait impl hidden by trait bound on function

[stephaneyfx]

I could have sworn I read a similar issue a couple of days/weeks ago but I could not find it back. #37138 seems related too.
Shoudn't this code compile? playground

struct A;
trait Foo<T> {}
impl Foo<i32> for A {}

fn f<T>(_: T) where A: Foo<T> {
    g(3i32);
    // Works:
    // g::<i32>(3);
}

fn g<T>(_: T) where A: Foo<T> {}

fn main() {
    f(3);
}

error[E0308]: mismatched types
 --> src/main.rs:6:7
  |
6 |     g(3i32);
  |       ^^^^ expected type parameter, found i32
  |
  = note: expected type `T`
             found type `i32`

error: aborting due to previous error

[stephaneyfx]

@stokhos, I looked at issues labeled with A-specialization and they seem to relate to the specialization feature in Rust. The example code in this issue does not make use of this feature, only of plain old trait impls. Is this label appropriate?

[Centril]

Seems like a bug?

Just to illustrate that this is not specific to i32 and having the same type variable names (tested on nightly, etc.):

#![crate_type = "lib"]

struct A;
struct B;
trait Foo<T> {}
impl Foo<B> for A {}

fn f<T0>(_: T0) where A: Foo<T0> {
    g::<_>(B); // expected type `T0`
}

fn g<T1>(_: T1) where A: Foo<T1> {}

But this works fine:

struct A;
struct B;
trait Foo<T> {}
impl Foo<B> for A {}

fn main() { g::<_>(B); }

fn g<T>(_: T) where A: Foo<T> {}

[stephaneyfx]

@Centril: As requested, I tried with older rust versions and the original example is rejected even by rust 1.0.0: https://godbolt.org/g/ECbAi4

[Centril]

@stephaneyfx Great, thank you ❤️; then at least there's no regression :)

[stephaneyfx]

This looks very much like #24066 and the original example not compiling seems to be intentional: see comments starting at #33108 (comment), especially from @eddyb and @aturon.