Cannot upcast dyn Trait in a way that reduces the number of associated types

[compiler-errors]

I tried this code:

#![feature(trait_upcasting)]

trait A: B {
    type Assoc;
}

trait B {}

fn upcast(a: &dyn A<Assoc = i32>) -> &dyn B { a }

fn main() {}

I expected to see it compile.

Instead, this happened:

error[E0308]: mismatched types
 --> src/main.rs:9:47
  |
9 | fn upcast(a: &dyn A<Assoc = i32>) -> &dyn B { a }
  |                                      ------   ^ expected trait `B`, found trait `A<Assoc = i32>`
  |                                      |
  |                                      expected `&dyn B` because of return type
  |
  = note: expected reference `&dyn B`
             found reference `&dyn A<Assoc = i32>`

The algorithm we use to do trait upcasting is incorrect, since it simply copies the existential associated types over:

rust/compiler/rustc_trait_selection/src/traits/select/confirmation.rs

Lines 933 to 937 in fd56162

	.chain(
	data_a
	.projection_bounds()
	.map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
	)

So if we upcast to a trait with fewer associated types (like B), then the subtyping we do here is wrong:

rust/compiler/rustc_trait_selection/src/traits/select/confirmation.rs

Lines 949 to 953 in fd56162

	let InferOk { obligations, .. } = self
	.infcx
	.at(&obligation.cause, obligation.param_env)
	.sup(DefineOpaqueTypes::No, target, source_trait)
	.map_err(|_| Unimplemented)?;

Since we require the list of existential trait bounds to be structurally compatible:

rust/compiler/rustc_middle/src/ty/relate.rs

Lines 689 to 698 in fd56162

	let mut a_v: Vec<_> = a.into_iter().collect();
	let mut b_v: Vec<_> = b.into_iter().collect();
	// `skip_binder` here is okay because `stable_cmp` doesn't look at binders
	a_v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
	a_v.dedup();
	b_v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
	b_v.dedup();
	if a_v.len() != b_v.len() {
	return Err(TypeError::ExistentialMismatch(expected_found(relation, a, b)));
	}