HIR ty lowering: Validate unbounds in trait object types

Only relevant to the internal feature `more_maybe_bounds`.

Author: fmease

compiler/rustc_hir_analysis/src/hir_ty_lowering/dyn_compatibility.rs

@@ -36,9 +36,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let mut user_written_bounds = Vec::new();
         let mut potential_assoc_types = Vec::new();
         for poly_trait_ref in hir_bounds.iter() {
-            if let hir::BoundPolarity::Maybe(_) = poly_trait_ref.modifiers.polarity {
-                continue;
-            }
             let result = self.lower_poly_trait_ref(
                 poly_trait_ref,
                 dummy_self,

tests/ui/traits/default_auto_traits/maybe-bounds-in-dyn-traits.rs

@@ -64,4 +64,8 @@ fn main() {
     x.leak_foo();
     //~^ ERROR the trait bound `dyn Trait: Leak` is not satisfied
     x.maybe_leak_foo();
+    // Ensure that we validate the generic args of maybe-bounds
+    let _: dyn Trait + ?Leak<(), Undefined = ()>;
+    //~^ ERROR trait takes 0 generic arguments but 1 generic argument was supplied
+    //~| ERROR associated type `Undefined` not found for `Leak`
 }

tests/ui/traits/default_auto_traits/maybe-bounds-in-dyn-traits.stderr

@@ -18,6 +18,27 @@ note: required by a bound in `Trait::leak_foo`
 LL |     fn leak_foo(&self) {}
    |     ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Trait::leak_foo`
 
-error: aborting due to 2 previous errors
+error[E0107]: trait takes 0 generic arguments but 1 generic argument was supplied
+  --> $DIR/maybe-bounds-in-dyn-traits.rs:68:25
+   |
+LL |     let _: dyn Trait + ?Leak<(), Undefined = ()>;
+   |                         ^^^^-------------------- help: remove the unnecessary generics
+   |                         |
+   |                         expected 0 generic arguments
+   |
+note: trait defined here, with 0 generic parameters
+  --> $DIR/maybe-bounds-in-dyn-traits.rs:44:12
+   |
+LL | auto trait Leak {}
+   |            ^^^^
+
+error[E0220]: associated type `Undefined` not found for `Leak`
+  --> $DIR/maybe-bounds-in-dyn-traits.rs:68:34
+   |
+LL |     let _: dyn Trait + ?Leak<(), Undefined = ()>;
+   |                                  ^^^^^^^^^ associated type `Undefined` not found
+
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0107, E0220, E0277.
+For more information about an error, try `rustc --explain E0107`.