Redundant Bound breaks const-generics Builders

[EsotericPyramid]

The following code produces 3 errors (at the bottom in the impl of Foo):

pub trait Builder {
    type Wrapped;

    // in my project this wraps something but that isn't needed for the bug
    fn wrap(&self, inner: ()) -> Self::Wrapped;
}
pub trait BuilderUnion<T: Builder>: Builder {
    type Union: Builder;

    fn union(self, other: T) -> Self::Union;
}

pub struct WrapperBuilder<const D: usize>;

impl<const D: usize> Builder for WrapperBuilder<D> {
    type Wrapped = Wrapper<D>;

    fn wrap(&self, _: ()) -> Self::Wrapped {
        Wrapper()
    }
}

impl<const D: usize> BuilderUnion<WrapperBuilder<D>> for WrapperBuilder<D> {
    type Union = Self;

    fn union(self, _: WrapperBuilder<D>) -> Self::Union {
        self
    }
}

// in my project this wraps something but that isn't needed for the bug
pub struct Wrapper<const D: usize>();

pub trait Bar {
    fn is_bar(&self) -> () {}
}

impl<const D: usize> Bar for Wrapper<D> {}

pub trait Foo<T> {
    fn foo(self, rhs: T);
}

impl<const D: usize> Foo<WrapperBuilder<D>> for WrapperBuilder<D>
where
    Self: BuilderUnion<Self>,
{
    // the error is here, each `is_bar` produces an error because Rust doesn't think Bar is impl'd 
    fn foo(self, rhs: WrapperBuilder<D>) {
        (self).wrap(()).is_bar();
        (rhs).wrap(()).is_bar();
        (self.union(rhs)).wrap(()).is_bar();
    }
}

I expected to see this happen: No errors produced

Instead, this happened: 1 error produced for each is_bar call because the type doesn't implement Bar (even though it does because it is ultimately just Wrapper<D>)

Extra Notes:

• removing the Self: BuilderUnion<Self> bound causes it to function correctly (even though its true in both cases and irrelevent to the first 2 calls)
• removing the const generics causes it to function correctly

Meta

rustc --version --verbose:

rustc 1.93.0 (254b59607 2026-01-19)
binary: rustc
commit-hash: 254b59607d4417e9dffbc307138ae5c86280fe4c
commit-date: 2026-01-19
host: aarch64-apple-darwin
release: 1.93.0
LLVM version: 21.1.8

(also fails on rustc 1.95.0-nightly (e96bb7e44 2026-01-27))

Backtrace

   Compiling bug_tests v0.1.0 ([...]/bug_tests)
error[E0599]: no method named `is_bar` found for associated type `<WrapperBuilder<D> as Builder>::Wrapped` in the current scope
  --> src/lib.rs:49:25
   |
49 |         (self).wrap(()).is_bar();
   |                         ^^^^^^ method not found in `<WrapperBuilder<D> as Builder>::Wrapped`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
note: `Bar` defines an item `is_bar`, perhaps you need to implement it
  --> src/lib.rs:34:1
   |
34 | pub trait Bar {
   | ^^^^^^^^^^^^^

error[E0599]: no method named `is_bar` found for associated type `<WrapperBuilder<D> as Builder>::Wrapped` in the current scope
  --> src/lib.rs:50:24
   |
50 |         (rhs).wrap(()).is_bar();
   |                        ^^^^^^ method not found in `<WrapperBuilder<D> as Builder>::Wrapped`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
note: `Bar` defines an item `is_bar`, perhaps you need to implement it
  --> src/lib.rs:34:1
   |
34 | pub trait Bar {
   | ^^^^^^^^^^^^^

error[E0599]: no method named `is_bar` found for associated type `<<WrapperBuilder<D> as BuilderUnion<WrapperBuilder<D>>>::Union as Builder>::Wrapped` in the current scope
  --> src/lib.rs:51:36
   |
51 |         (self.union(rhs)).wrap(()).is_bar();
   |                                    ^^^^^^ method not found in `<<WrapperBuilder<D> as BuilderUnion<WrapperBuilder<D>>>::Union as Builder>::Wrapped`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
note: `Bar` defines an item `is_bar`, perhaps you need to implement it
  --> src/lib.rs:34:1
   |
34 | pub trait Bar {
   | ^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0599`.
error: could not compile `bug_tests` (lib) due to 3 previous errors