Auto merge of rust-lang#114080 - compiler-errors:negative, r=spastorino

Don't treat negative trait predicates as always knowable

We don't need this. It was added in rust-lang#90104 but I don't really know why. It's not sound afaict -- negative trait predicates need the same coherence-ambiguity/orphan check rules as positive ones.

r? `@lcnr`

cc `@spastorino,` do you remember why?

Author: bors

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1487,7 +1487,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
         debug!("is_knowable(intercrate={:?})", self.is_intercrate());
 
-        if !self.is_intercrate() || stack.obligation.polarity() == ty::ImplPolarity::Negative {
+        if !self.is_intercrate() {
             return Ok(());
         }