Treat identity region outlives as true

compiler/rustc_type_ir/src/region_constraint.rs

@@ -681,6 +681,10 @@ fn pull_region_outlives_constraints_out_of_universe<
             constraint
         }
         RegionOutlives(region_1, region_2, ()) => {
+            if region_1 == region_2 {
+                return RegionConstraint::new_true();
+            }
+
             let region_1_u = max_universe(infcx, region_1);
             let region_2_u = max_universe(infcx, region_2);
 

tests/ui/assumptions_on_binders/identity_region_outlives.rs

@@ -0,0 +1,29 @@
+//@ compile-flags: -Zassumptions-on-binders
+//@ check-pass
+
+// Regression test for #161733.
+// Previously we didn't properly handle identity region outlives
+// and it was mistakenly considered as false region constraint.
+// The closure is needed to register the type outlives obligation
+// in borrowck.
+
+#![feature(test_binder_constraints)]
+
+fn foo<T>()
+where
+    for<'a> &'a T: 'a,
+{
+    || {};
+}
+
+core::test_binder_constraints! {
+    impl<T> {
+        forall<'a> where T: 'a {
+            'a: 'a,
+            T: 'a,
+        } expect {
+        }
+    }
+}
+
+fn main() {}