From 38ec810c37439a6a3742d85b950978d904b4f826 Mon Sep 17 00:00:00 2001
From: Alan Egerton <eggyal@gmail.com>
Date: Wed, 8 Feb 2023 19:55:50 +0000
Subject: [PATCH] Do not assemble supertraits for trait aliases

---
 compiler/rustc_hir_typeck/src/method/probe.rs | 48 ++++++++++++-------
 ...ssue-107747-do-not-assemble-supertraits.rs | 21 ++++++++
 2 files changed, 52 insertions(+), 17 deletions(-)
 create mode 100644 tests/ui/traits/alias/issue-107747-do-not-assemble-supertraits.rs

diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 9ab29a6778fc9..d6b054d9626c3 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -951,24 +951,38 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         let trait_ref = self.tcx.mk_trait_ref(trait_def_id, trait_substs);
 
         if self.tcx.is_trait_alias(trait_def_id) {
-            // For trait aliases, assume all supertraits are relevant.
-            let bounds = iter::once(ty::Binder::dummy(trait_ref));
-            self.elaborate_bounds(bounds, |this, new_trait_ref, item| {
-                let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);
+            // For trait aliases, recursively assume all explicitly named traits are relevant
+            for expansion in traits::expand_trait_aliases(
+                self.tcx,
+                iter::once((ty::Binder::dummy(trait_ref), self.span)),
+            ) {
+                let bound_trait_ref = expansion.trait_ref();
+                for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
+                    if !self.has_applicable_self(&item) {
+                        self.record_static_candidate(CandidateSource::Trait(
+                            bound_trait_ref.def_id(),
+                        ));
+                    } else {
+                        let new_trait_ref = self.erase_late_bound_regions(bound_trait_ref);
 
-                let (xform_self_ty, xform_ret_ty) =
-                    this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
-                this.push_candidate(
-                    Candidate {
-                        xform_self_ty,
-                        xform_ret_ty,
-                        item,
-                        import_ids: import_ids.clone(),
-                        kind: TraitCandidate(new_trait_ref),
-                    },
-                    false,
-                );
-            });
+                        let (xform_self_ty, xform_ret_ty) = self.xform_self_ty(
+                            &item,
+                            new_trait_ref.self_ty(),
+                            new_trait_ref.substs,
+                        );
+                        self.push_candidate(
+                            Candidate {
+                                xform_self_ty,
+                                xform_ret_ty,
+                                item,
+                                import_ids: import_ids.clone(),
+                                kind: TraitCandidate(new_trait_ref),
+                            },
+                            false,
+                        );
+                    }
+                }
+            }
         } else {
             debug_assert!(self.tcx.is_trait(trait_def_id));
             if self.tcx.trait_is_auto(trait_def_id) {
diff --git a/tests/ui/traits/alias/issue-107747-do-not-assemble-supertraits.rs b/tests/ui/traits/alias/issue-107747-do-not-assemble-supertraits.rs
new file mode 100644
index 0000000000000..9b41a8096c4e5
--- /dev/null
+++ b/tests/ui/traits/alias/issue-107747-do-not-assemble-supertraits.rs
@@ -0,0 +1,21 @@
+// Regression test for #107747: methods from trait alias supertraits were brought into scope
+//
+// check-pass
+
+#![feature(trait_alias)]
+
+use std::fmt;
+
+trait Foo: fmt::Debug {}
+trait Bar = Foo;
+
+#[derive(Debug)]
+struct Qux(bool);
+
+impl fmt::Display for Qux {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+fn main() {}