What is type conversion in parameter for?

[harrypunk]

Hi,
I was reading the source code and came across the type conversion in function signatures:
https://github.com/bevyengine/bevy/blob/97d8e4e1793ede3df8c77ed44736e800b38ff7a4/crates/bevy_ecs/src/system/into_system.rs

pub struct FunctionSystem<In, Out, Param, Marker, F>
where
    Param: SystemParam,
{
    config: Option<<Param::Fetch as SystemParamState>::Config>,
}

I just wonder what the conversion does here, Why not SystemParamState::Config?

[bjorn3]

The type Param::Fetch implements the trait SystemParamState which has an associated type Config. The as SystemParamState indicates that the associated type Config should be looked for in the implementation of SystemParamState for Param::Fetch.