From 15e6565646549a05ef5be0d7a376a96d64ac6463 Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Fri, 13 Mar 2026 17:05:32 +0100
Subject: [PATCH] Implement fix in `try_promote_type_test_subject` + add a
 test.

Fix in question: We only propagate `T: 'ub` requirements when 'ub actually outlives
the lower bounds of the type test. If none of the non-local upper bounds outlive it,
then we propagate all of them.
---
 .../rustc_borrowck/src/region_infer/mod.rs    | 28 ++++++++++++++++++-
 .../type-test-issue-154267.rs                 | 25 +++++++++++++++++
 2 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/nll/closure-requirements/type-test-issue-154267.rs

diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 5cdda777723b3..06b9dec7d0412 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -713,9 +713,35 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         for ur in self.scc_values.universal_regions_outlived_by(r_scc) {
             found_outlived_universal_region = true;
             debug!("universal_region_outlived_by ur={:?}", ur);
-            let non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
+            let mut non_local_ub = self.universal_region_relations.non_local_upper_bounds(ur);
             debug!(?non_local_ub);
 
+            // We don't need to propagate every `T: 'ub` for soundness:
+            // Say we have the following outlives constraints given (`'b: 'a` == `a -> b`):
+            //
+            // a
+            //  \
+            //   -> c
+            //  /
+            // b
+            //
+            // And we are doing the type test `T: 'a`.
+            //
+            // The `lower_bound_universal_regions` of `'a` are `['a, 'c]`. The `upper_bounds` of `'a`
+            // is `['a]`, so we propagate `T: 'a`.
+            // The `upper_bounds` of `'c` are `['a, 'b]`, propagating `T: 'a` is correct;
+            // `T: 'b` is redundant because it provides no value to proving `T: 'a`.
+            //
+            // So we filter the set of upper_bounds to regions already outliving `lower_bound`,
+            // but if this subset is empty, we fall back to the original one.
+            let subset: Vec<_> = non_local_ub
+                .iter()
+                .copied()
+                .filter(|ub| self.eval_outlives(*ub, lower_bound))
+                .collect();
+            non_local_ub = if subset.is_empty() { non_local_ub } else { subset };
+            debug!(?non_local_ub, "upper_bounds after filtering");
+
             // This is slightly too conservative. To show T: '1, given `'2: '1`
             // and `'3: '1` we only need to prove that T: '2 *or* T: '3, but to
             // avoid potential non-determinism we approximate this by requiring
diff --git a/tests/ui/nll/closure-requirements/type-test-issue-154267.rs b/tests/ui/nll/closure-requirements/type-test-issue-154267.rs
new file mode 100644
index 0000000000000..836802ecd481e
--- /dev/null
+++ b/tests/ui/nll/closure-requirements/type-test-issue-154267.rs
@@ -0,0 +1,25 @@
+//@ check-pass
+// This test checks that the compiler doesn't propagate `T: 'b` during the `T: 'a` type test.
+// If it did, it would fail to compile, even though the program is sound.
+
+struct Arg<'a: 'c, 'b: 'c, 'c, T> {
+    field: *mut (&'a (), &'b (), &'c (), T),
+}
+
+impl<'a, 'b, 'c, T> Arg<'a, 'b, 'c, T> {
+    fn constrain(self)
+    where
+        T: 'a,
+    {
+    }
+}
+
+fn takes_closure<'a, 'b, T>(_: impl for<'c> FnOnce(Arg<'a, 'b, 'c, T>)) {}
+
+// We have `'a: 'c` and `'b: 'c`, requiring `T: 'a` in `constrain` should not need
+// `T: 'b` here.
+fn error<'a, 'b, T: 'a>() {
+    takes_closure::<'a, 'b, T>(|arg| arg.constrain());
+}
+
+fn main() {}