defaults on `GATs` emit an error but function correctlyish

[BoxyUwU]

I tried this code: playground

#![feature(generic_associated_types)]
#![allow(invalid_type_param_default)]

trait Trait {
    type Assoc<T = u32>;
}

impl Trait for () {
    type Assoc<T = u32> = u64; 
}

impl Trait for u32 {
    type Assoc<T = u32> = T;
}

trait Other {}
impl Other for u32 {}

fn foo<T>()
where
    T: Trait<Assoc = u32>, 
    T::Assoc: Other {
        
    }
    
fn main() {
    // errors
    foo::<()>();
    // works
    foo::<u32>();
}

I expected to see this happen: It not work correctly as if you remove #![allow(invalid_type_param_default)] it shouts at you saying its unsupported and will be a hard error one day.

Instead, this happened: defaults on GATs seem to function perfectly fine

Meta

version from playground:

 1.64.0-nightly
(2022-07-12 1c7b36d4db582cb47513)

cc @jackh726 this is probably should either be allowed or a hard error before stabilisation?

[lcnr]

one issue is that we ignore defaults on the gat in impls

#![feature(generic_associated_types)]
#![allow(invalid_type_param_default)]

trait Trait {
    type Assoc<T = u32>;
}

impl Trait for () {
    type Assoc<T> = u32; 
}

impl Trait for u32 {
    type Assoc<T = u32> = T;
}

trait Trait2 {
    type Assoc<T>;
}

impl Trait2 for () {
    type Assoc<T> = u32; 
}

impl Trait2 for u32 {
    type Assoc<T = u32> = T;
}


fn foo<T>() {}
    
fn main() {
    foo::<<() as Trait>::Assoc>(); // ok
    foo::<<u32 as Trait>::Assoc>(); // ok
    
    foo::<<() as Trait2>::Assoc>(); // err
    foo::<<u32 as Trait2>::Assoc>(); // err
}

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium