#[derive] picks incorrect bounds when associated types are involved

[lambda-fairy]

Minimal example:

trait Foo {
    type T;
}

#[derive(Copy)]
struct PairFoo<F: Foo>(pub F::T, pub F::T);

fn main() {}

rustc -Z unstable-options --pretty=expanded yields:

#![feature(no_std)]
#![no_std]
#[prelude_import]
use std::prelude::v1::*;
#[macro_use]
extern crate "std" as std;
trait Foo {
    type
    T;
}

struct PairFoo<F: Foo>(pub F::T, pub F::T);
#[automatically_derived]
impl <F: ::std::marker::Copy + Foo> ::std::marker::Copy for PairFoo<F> { }

fn main() { }

This is wrong, because the bound shouldn't be F: Copy but <F as Foo>::T: Copy instead.

[lambda-fairy]

Duplicate of #23575, whoopsies