auto trait incorrectly "unimplemented" by blanket negative impl

[eggyal]

I tried this code (playground):

#![feature(auto_traits, negative_impls)]

auto trait Blob {}

trait NotBlob {}
impl<B: NotBlob> !Blob for B {}

fn assert_blob(_: impl Blob) {}
fn main() {
    assert_blob(())
}

I expected the code to compile, because Blob is auto-implemented for all types except those for which NotBlob has been implemented (there aren't any); instead, the compiler complains that Blob is not implemented for the unit type ():

error[E0277]: the trait bound `(): Blob` is not satisfied
  --> src/main.rs:10:17
   |
10 |     assert_blob(())
   |     ----------- ^^ the trait `Blob` is not implemented for `()`
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `assert_blob`
  --> src/main.rs:8:24
   |
8  | fn assert_blob(_: impl Blob) {}
   |                        ^^^^ required by this bound in `assert_blob`

For more information about this error, try `rustc --explain E0277`.

Meta

rustc --version --verbose:

rustc 1.69.0 (5243ea5c2 2023-02-20)
binary: rustc
commit-hash: 5243ea5c29b136137c36bd773e5baa663790e097
commit-date: 2023-02-20
host: x86_64-unknown-linux-gnu
release: 1.69.0
LLVM version: 15.0.7

@rustbot label +F-auto_traits +F-negative_impls

[compiler-errors]

I think this is necessary for coherence soundness? When considering whether to use an auto trait bound's "implicit" auto implementation, or a user-defined impl (whether negative or positive), we only consider the impl's header and not the where clauses. So to trait selection, it looks like impl<T> !Blob for T and we don't consider the nested where clauses until later.

Anyways, this is a duplicate of #46813 I think!