Don't drop region constraints that come from plugging infer regions with placeholders

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -415,13 +415,6 @@ fn impl_intersection_has_negative_obligation(
         return false;
     };
 
-    plug_infer_with_placeholders(
-        infcx,
-        root_universe,
-        (impl1_header.impl_args, impl2_header.impl_args),
-    );
-    let param_env = infcx.resolve_vars_if_possible(param_env);
-
     // FIXME(with_negative_coherence): the infcx has constraints from equating
     // the impl headers. We should use these constraints as assumptions, not as
     // requirements, when proving the negated where clauses below.
@@ -429,6 +422,13 @@ fn impl_intersection_has_negative_obligation(
     drop(infcx.take_registered_region_obligations());
     drop(infcx.take_and_reset_region_constraints());
 
+    plug_infer_with_placeholders(
+        infcx,
+        root_universe,
+        (impl1_header.impl_args, impl2_header.impl_args),
+    );
+    let param_env = infcx.resolve_vars_if_possible(param_env);
+
     util::elaborate(tcx, tcx.predicates_of(impl2_def_id).instantiate(tcx, impl2_header.impl_args))
         .any(|(clause, _)| try_prove_negated_where_clause(infcx, clause, param_env))
 }

tests/ui/coherence/coherence-negative-outlives-lifetimes.rs

@@ -1,5 +1,9 @@
 // revisions: stock with_negative_coherence
+
 //[with_negative_coherence] known-bug: unknown
+// Ideally this would work, but we don't use `&'a T` to imply that `T: 'a`
+// which is required for `&'a T: !MyPredicate` to hold. This is similar to the
+// test `negative-coherence-placeholder-region-constraints-on-unification.explicit.stderr`
 
 #![feature(negative_impls)]
 #![cfg_attr(with_negative_coherence, feature(with_negative_coherence))]

tests/ui/coherence/coherence-negative-outlives-lifetimes.stock.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `MyTrait<'_>` for type `&_`
-  --> $DIR/coherence-negative-outlives-lifetimes.rs:14:1
+  --> $DIR/coherence-negative-outlives-lifetimes.rs:18:1
    |
 LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
    | ---------------------------------------------- first implementation here

tests/ui/coherence/coherence-negative-outlives-lifetimes.with_negative_coherence.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `MyTrait<'_>` for type `&_`
-  --> $DIR/coherence-negative-outlives-lifetimes.rs:14:1
+  --> $DIR/coherence-negative-outlives-lifetimes.rs:18:1
    |
 LL | impl<'a, T: MyPredicate<'a>> MyTrait<'a> for T {}
    | ---------------------------------------------- first implementation here