From 7c5ea7ffd648cb1170510a7b50b0294a30891ffd Mon Sep 17 00:00:00 2001
From: Ruiyang Wang <rynewang@users.noreply.github.com>
Date: Sat, 7 Feb 2026 17:57:53 +0000
Subject: [PATCH] Fix bound var resolution for trait aliases

Handle DefKind::TraitAlias in resolve_bound_vars so that associated
item constraints and return type notation work through trait aliases.
---
 .../src/collect/resolve_bound_vars.rs              |  5 +++--
 .../return-type-notation/trait-alias.rs            | 13 +++++++++++++
 .../trait-alias-bound-vars.rs                      | 14 ++++++++++++++
 3 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs
 create mode 100644 tests/ui/associated-type-bounds/trait-alias-bound-vars.rs

diff --git a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
index 26f79d374075b..d4a01d26a1fc4 100644
--- a/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
+++ b/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs
@@ -1695,7 +1695,8 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
                 | DefKind::Union
                 | DefKind::Enum
                 | DefKind::TyAlias
-                | DefKind::Trait,
+                | DefKind::Trait
+                | DefKind::TraitAlias,
                 def_id,
             ) if depth == 0 => Some(def_id),
             _ => None,
@@ -1865,7 +1866,7 @@ impl<'a, 'tcx> BoundVarContext<'a, 'tcx> {
             if constraint.gen_args.parenthesized == hir::GenericArgsParentheses::ReturnTypeNotation
             {
                 let bound_vars = if let Some(type_def_id) = type_def_id
-                    && self.tcx.def_kind(type_def_id) == DefKind::Trait
+                    && let DefKind::Trait | DefKind::TraitAlias = self.tcx.def_kind(type_def_id)
                     && let Some((mut bound_vars, assoc_fn)) = BoundVarContext::supertrait_hrtb_vars(
                         self.tcx,
                         type_def_id,
diff --git a/tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs b/tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs
new file mode 100644
index 0000000000000..88f60a72afa19
--- /dev/null
+++ b/tests/ui/associated-type-bounds/return-type-notation/trait-alias.rs
@@ -0,0 +1,13 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/152158>.
+//@ check-pass
+#![feature(return_type_notation, trait_alias)]
+
+trait Tr {
+    fn f() -> impl Sized;
+}
+
+trait Al = Tr;
+
+fn f<T: Al<f(..): Copy>>() {}
+
+fn main() {}
diff --git a/tests/ui/associated-type-bounds/trait-alias-bound-vars.rs b/tests/ui/associated-type-bounds/trait-alias-bound-vars.rs
new file mode 100644
index 0000000000000..4644cee210083
--- /dev/null
+++ b/tests/ui/associated-type-bounds/trait-alias-bound-vars.rs
@@ -0,0 +1,14 @@
+// Check that we're successfully collecting bound vars behind trait aliases.
+// Regression test for <https://github.com/rust-lang/rust/issues/152244>.
+//@ check-pass
+//@ needs-rustc-debug-assertions
+#![feature(trait_alias)]
+
+trait A<'a> { type X; }
+trait B: for<'a> A<'a> {}
+trait C = B;
+
+fn f<T>() where T: C<X: Copy> {}
+fn g<T>() where T: C<X: for<'r> A<'r>> {}
+
+fn main() {}