Do not consider a T: !Sized candidate to satisfy a T: !MetaSized obligation.

[zachs18]

This example should fail to compile (and does under this PR, with the old and new solvers), but currently compiles successfully (playground), because (IIUC) the old solver's lazily_elaborate_sizedness_candidate/callers and the new solver's TraitPredicate::fast_reject_assumption/match_assumption consider a T: _ Sized candidate to satisfy a T: _ MetaSized obligation, for either polarity _, when that should only hold for positive polarity.

#![feature(negative_bounds)]
#![feature(sized_hierarchy)]

use std::marker::MetaSized;

fn foo<T: !MetaSized>() {}

fn bar<T: !Sized + MetaSized>() {
    foo::<T>();
    //~^ ERROR the trait bound `T: !MetaSized` is not satisfied // error under this PR
}

Only observable with the internal-only feature(negative_bounds), so might just be "wontfix".

This example is added as a test in this PR (as well as testing that foo<()> and foo<str> are disallowed for fn foo<T: !MetaSized).

cc @davidtwco for feature(sized_hierarchy)

Maybe similar to 91c53c9 from #143307

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[davidtwco]

I don't know what our plans are for negative_bounds but this makes sense to me.

@bors r+ rollup

[bors]

📌 Commit c7cd1b3 has been approved by davidtwco

It is now in the queue for this repository.