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?