Default impls don't substitute type parameters properly in rustdoc

[Manishearth]

For example, on http://doc.rust-lang.org/nightly/std/primitive.slice.html, we have

impl<T> Repr<Slice<T>> for [T]
fn repr(&self) -> T

however it should actually be

impl<T> Repr<Slice<T>> for [T]
fn repr(&self) -> Option<T>

since Repr is

pub trait Repr<T> {
    fn repr() -> T { ... }
}

Out of tree example:

pub trait Repr<T> {
   fn repr() -> T {unimplemented!()}
}
pub struct Foo;
impl<T> Repr<Option<T>> for Foo {}

It doesn't even need the type parameter:

pub trait Repr<T> {
   fn repr() -> T {unimplemented!()}
}
pub struct Foo;
impl Repr<u8> for Foo {}

Looks like rustdoc is picking up the signature from the default impl.

[Mark-Simulacrum]

Closing in favor of #14072 which has more information and discussion.