derive generates incorrect bounds for associated types

[IsaacBreen]

I think there's a bug causing incorrect derive behaviour for a field whose type is an associated type extracted from a generic parameter.

trait AssociatedWithDefaultable {
    type Defaultable: Default;  // (1) `Defaultable` implements `Default`.
}

// (2) Expectation: `StoreAssociatedDefaultable` implements `Default` due to (3).
#[derive(Default)]
struct StoreAssociatedDefaultable<AWC: AssociatedWithDefaultable> {
    defaultable: AWC::Defaultable,  // (3) `Defaultable` implements `Default` due to (1).
}

fn get_default<A: AssociatedWithDefaultable>() -> StoreAssociatedDefaultable<A> {
    Default::default()
}

error[E0277]: the trait bound `A: Default` is not satisfied
  --> src/main.rs:12:5
   |
12 |     Default::default()
   |     ^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `A`, which is required by `StoreAssociatedDefaultable<A>: Default`
   |
note: required for `StoreAssociatedDefaultable<A>` to implement `Default`
  --> src/main.rs:6:10
   |
6  | #[derive(Default)]
   |          ^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider further restricting this bound
   |
11 | fn get_default<A: AssociatedWithDefaultable + std::default::Default>() -> StoreAssociatedDefaultable<A> {
   |                                             +++++++++++++++++++++++

The macro expansion makes the problem clearer.

struct StoreAssociatedDefaultable<AWC: AssociatedWithDefaultable> {
    defaultable: AWC::Defaultable,
}
#[automatically_derived]
impl<AWC: ::core::default::Default + AssociatedWithDefaultable> ::core::default::Default
//        ^^^^^^^^^^^^^^^^^^^^^^^^ incorrect
for StoreAssociatedDefaultable<AWC>
where
    AWC::Defaultable: ::core::default::Default,
//                    ^^^^^^^^^^^^^^^^^^^^^^^^ correct but unnecessary
{
    #[inline]
    fn default() -> StoreAssociatedDefaultable<AWC> {
        StoreAssociatedDefaultable {
            defaultable: ::core::default::Default::default(),
        }
    }
}

It seems to confuse the associated type with the generic that carries it, causing it to add the bound AWC: ::core::default::Default unexpectedly.

It also adds AWC::Defaultable: ::core::default::Default, which is consistent but superfluous.

Link to playground

> rustc --version --verbose
rustc 1.78.0-nightly (3246e7951 2024-02-19)
binary: rustc
commit-hash: 3246e79513cb89ddbfc0f21cb5a877e5b321dcc5
commit-date: 2024-02-19
host: aarch64-apple-darwin
release: 1.78.0-nightly
LLVM version: 18.1.0

[krtab]

I think this is a more intricate version of

use core::marker::PhantomData;

#[derive(Default)]
struct F<A> {
    ph: PhantomData<A>
}

struct X {}

fn f() -> F<X> {
    F::default()
}

which is expected #26925