diff --git a/src/librustc/hir/map/def_collector.rs b/src/librustc/hir/map/def_collector.rs
index bc0a64ae7c5d8..c9b4b2bb99717 100644
--- a/src/librustc/hir/map/def_collector.rs
+++ b/src/librustc/hir/map/def_collector.rs
@@ -120,10 +120,10 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
         let def_data = match i.node {
             ItemKind::Impl(..) => DefPathData::Impl,
             ItemKind::Trait(..) => DefPathData::Trait(i.ident.as_interned_str()),
+            ItemKind::TraitAlias(..) => DefPathData::TraitAlias(i.ident.as_interned_str()),
             ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) |
-            ItemKind::TraitAlias(..) | ItemKind::Existential(..) |
-            ItemKind::ExternCrate(..) | ItemKind::ForeignMod(..) | ItemKind::Ty(..) =>
-                DefPathData::TypeNs(i.ident.as_interned_str()),
+            ItemKind::Existential(..) | ItemKind::ExternCrate(..) | ItemKind::ForeignMod(..) |
+            ItemKind::Ty(..) => DefPathData::TypeNs(i.ident.as_interned_str()),
             ItemKind::Mod(..) if i.ident == keywords::Invalid.ident() => {
                 return visit::walk_item(self, i);
             }
diff --git a/src/librustc/hir/map/definitions.rs b/src/librustc/hir/map/definitions.rs
index 6b707dd2dcc80..1b7445199475c 100644
--- a/src/librustc/hir/map/definitions.rs
+++ b/src/librustc/hir/map/definitions.rs
@@ -373,7 +373,9 @@ pub enum DefPathData {
     /// GlobalMetaData identifies a piece of crate metadata that is global to
     /// a whole crate (as opposed to just one item). GlobalMetaData components
     /// are only supposed to show up right below the crate root.
-    GlobalMetaData(InternedString)
+    GlobalMetaData(InternedString),
+    /// A trait alias.
+    TraitAlias(InternedString),
 }
 
 #[derive(Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord, Debug,
@@ -615,6 +617,7 @@ impl DefPathData {
         match *self {
             TypeNs(name) |
             Trait(name) |
+            TraitAlias(name) |
             AssocTypeInTrait(name) |
             AssocTypeInImpl(name) |
             AssocExistentialInImpl(name) |
@@ -642,6 +645,7 @@ impl DefPathData {
         let s = match *self {
             TypeNs(name) |
             Trait(name) |
+            TraitAlias(name) |
             AssocTypeInTrait(name) |
             AssocTypeInImpl(name) |
             AssocExistentialInImpl(name) |
diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index aaef1c722be96..4fc50bce272fa 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -10,6 +10,7 @@ pub use self::PrimTy::*;
 pub use self::UnOp::*;
 pub use self::UnsafeSource::*;
 
+use errors::FatalError;
 use hir::def::Def;
 use hir::def_id::{DefId, DefIndex, LocalDefId, CRATE_DEF_INDEX};
 use util::nodemap::{NodeMap, FxHashSet};
@@ -2055,6 +2056,20 @@ pub struct TraitRef {
     pub hir_ref_id: HirId,
 }
 
+impl TraitRef {
+    /// Get the `DefId` of the referenced trait. It _must_ actually be a trait or trait alias.
+    pub fn trait_def_id(&self) -> DefId {
+        match self.path.def {
+            Def::Trait(did) => did,
+            Def::TraitAlias(did) => did,
+            Def::Err => {
+                FatalError.raise();
+            }
+            _ => unreachable!(),
+        }
+    }
+}
+
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct PolyTraitRef {
     /// The `'a` in `<'a> Foo<&'a T>`
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index 05a6cce895d1f..9a0610f45dec2 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -2198,7 +2198,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
 
         let def_id = obligation.predicate.def_id();
 
-        if ty::is_trait_alias(self.tcx(), def_id) {
+        if self.tcx().is_trait_alias(def_id) {
             candidates.vec.push(TraitAliasCandidate(def_id.clone()));
         }
 
diff --git a/src/librustc/ty/item_path.rs b/src/librustc/ty/item_path.rs
index 417e14054d24f..9328de4f6a0a1 100644
--- a/src/librustc/ty/item_path.rs
+++ b/src/librustc/ty/item_path.rs
@@ -311,6 +311,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             data @ DefPathData::Misc |
             data @ DefPathData::TypeNs(..) |
             data @ DefPathData::Trait(..) |
+            data @ DefPathData::TraitAlias(..) |
             data @ DefPathData::AssocTypeInTrait(..) |
             data @ DefPathData::AssocTypeInImpl(..) |
             data @ DefPathData::AssocExistentialInImpl(..) |
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index f1b36e8def88b..dd315cf42ce7e 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -3170,18 +3170,6 @@ pub fn is_impl_trait_defn(tcx: TyCtxt<'_, '_, '_>, def_id: DefId) -> Option<DefI
     None
 }
 
-/// Returns `true` if `def_id` is a trait alias.
-pub fn is_trait_alias(tcx: TyCtxt<'_, '_, '_>, def_id: DefId) -> bool {
-    if let Some(node_id) = tcx.hir().as_local_node_id(def_id) {
-        if let Node::Item(item) = tcx.hir().get(node_id) {
-            if let hir::ItemKind::TraitAlias(..) = item.node {
-                return true;
-            }
-        }
-    }
-    false
-}
-
 /// See `ParamEnv` struct definition for details.
 fn param_env<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                        def_id: DefId)
diff --git a/src/librustc/ty/util.rs b/src/librustc/ty/util.rs
index e989ef823e979..75fc0f716a2f6 100644
--- a/src/librustc/ty/util.rs
+++ b/src/librustc/ty/util.rs
@@ -526,6 +526,15 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    /// True if `def_id` refers to a trait alias (i.e., `trait Foo = ...;`).
+    pub fn is_trait_alias(self, def_id: DefId) -> bool {
+        if let DefPathData::TraitAlias(_) = self.def_key(def_id).disambiguated_data.data {
+            true
+        } else {
+            false
+        }
+    }
+
     /// True if this def-id refers to the implicit constructor for
     /// a tuple struct like `struct Foo(u32)`.
     pub fn is_struct_constructor(self, def_id: DefId) -> bool {
diff --git a/src/librustc/util/ppaux.rs b/src/librustc/util/ppaux.rs
index 04e571863d42f..51e9192cd290d 100644
--- a/src/librustc/util/ppaux.rs
+++ b/src/librustc/util/ppaux.rs
@@ -409,6 +409,7 @@ impl PrintContext {
                     DefPathData::AssocTypeInImpl(_) |
                     DefPathData::AssocExistentialInImpl(_) |
                     DefPathData::Trait(_) |
+                    DefPathData::TraitAlias(_) |
                     DefPathData::Impl |
                     DefPathData::TypeNs(_) => {
                         break;
diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index dc8db5be5820a..1f07e8f478b5a 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -418,6 +418,7 @@ impl<'tcx> EntryKind<'tcx> {
             EntryKind::Mod(_) => Def::Mod(did),
             EntryKind::Variant(_) => Def::Variant(did),
             EntryKind::Trait(_) => Def::Trait(did),
+            EntryKind::TraitAlias(_) => Def::TraitAlias(did),
             EntryKind::Enum(..) => Def::Enum(did),
             EntryKind::MacroDef(_) => Def::Macro(did, MacroKind::Bang),
             EntryKind::ForeignType => Def::ForeignTy(did),
@@ -520,17 +521,26 @@ impl<'a, 'tcx> CrateMetadata {
     }
 
     pub fn get_trait_def(&self, item_id: DefIndex, sess: &Session) -> ty::TraitDef {
-        let data = match self.entry(item_id).kind {
-            EntryKind::Trait(data) => data.decode((self, sess)),
-            _ => bug!(),
-        };
-
-        ty::TraitDef::new(self.local_def_id(item_id),
-                          data.unsafety,
-                          data.paren_sugar,
-                          data.has_auto_impl,
-                          data.is_marker,
-                          self.def_path_table.def_path_hash(item_id))
+        match self.entry(item_id).kind {
+            EntryKind::Trait(data) => {
+                let data = data.decode((self, sess));
+                ty::TraitDef::new(self.local_def_id(item_id),
+                                  data.unsafety,
+                                  data.paren_sugar,
+                                  data.has_auto_impl,
+                                  data.is_marker,
+                                  self.def_path_table.def_path_hash(item_id))
+            },
+            EntryKind::TraitAlias(_) => {
+                ty::TraitDef::new(self.local_def_id(item_id),
+                                  hir::Unsafety::Normal,
+                                  false,
+                                  false,
+                                  false,
+                                  self.def_path_table.def_path_hash(item_id))
+            },
+            _ => bug!("def-index does not refer to trait or trait alias"),
+        }
     }
 
     fn get_variant(&self,
@@ -615,10 +625,13 @@ impl<'a, 'tcx> CrateMetadata {
                                 item_id: DefIndex,
                                 tcx: TyCtxt<'a, 'tcx, 'tcx>)
                                 -> ty::GenericPredicates<'tcx> {
-        match self.entry(item_id).kind {
-            EntryKind::Trait(data) => data.decode(self).super_predicates.decode((self, tcx)),
-            _ => bug!(),
-        }
+        let super_predicates = match self.entry(item_id).kind {
+            EntryKind::Trait(data) => data.decode(self).super_predicates,
+            EntryKind::TraitAlias(data) => data.decode(self).super_predicates,
+            _ => bug!("def-index does not refer to trait or trait alias"),
+        };
+
+        super_predicates.decode((self, tcx))
     }
 
     pub fn get_generics(&self,
@@ -1014,7 +1027,8 @@ impl<'a, 'tcx> CrateMetadata {
         }
         def_key.parent.and_then(|parent_index| {
             match self.entry(parent_index).kind {
-                EntryKind::Trait(_) => Some(self.local_def_id(parent_index)),
+                EntryKind::Trait(_) |
+                EntryKind::TraitAlias(_) => Some(self.local_def_id(parent_index)),
                 _ => None,
             }
         })
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 2de1637fb0d9d..afb0218501d18 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1128,8 +1128,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
 
                 EntryKind::Impl(self.lazy(&data))
             }
-            hir::ItemKind::Trait(..) |
-            hir::ItemKind::TraitAlias(..) => {
+            hir::ItemKind::Trait(..) => {
                 let trait_def = tcx.trait_def(def_id);
                 let data = TraitData {
                     unsafety: trait_def.unsafety,
@@ -1141,6 +1140,13 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
 
                 EntryKind::Trait(self.lazy(&data))
             }
+            hir::ItemKind::TraitAlias(..) => {
+                let data = TraitAliasData {
+                    super_predicates: self.lazy(&tcx.super_predicates_of(def_id)),
+                };
+
+                EntryKind::TraitAlias(self.lazy(&data))
+            }
             hir::ItemKind::ExternCrate(_) |
             hir::ItemKind::Use(..) => bug!("cannot encode info for item {:?}", item),
         };
@@ -1214,6 +1220,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
                 hir::ItemKind::Impl(..) |
                 hir::ItemKind::Existential(..) |
                 hir::ItemKind::Trait(..) => Some(self.encode_generics(def_id)),
+                hir::ItemKind::TraitAlias(..) => Some(self.encode_generics(def_id)),
                 _ => None,
             },
             predicates: match item.node {
@@ -1226,7 +1233,8 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
                 hir::ItemKind::Union(..) |
                 hir::ItemKind::Impl(..) |
                 hir::ItemKind::Existential(..) |
-                hir::ItemKind::Trait(..) => Some(self.encode_predicates(def_id)),
+                hir::ItemKind::Trait(..) |
+                hir::ItemKind::TraitAlias(..) => Some(self.encode_predicates(def_id)),
                 _ => None,
             },
 
@@ -1236,7 +1244,8 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             // hack. (No reason not to expand it in the future if
             // necessary.)
             predicates_defined_on: match item.node {
-                hir::ItemKind::Trait(..) => Some(self.encode_predicates_defined_on(def_id)),
+                hir::ItemKind::Trait(..) |
+                hir::ItemKind::TraitAlias(..) => Some(self.encode_predicates_defined_on(def_id)),
                 _ => None, // not *wrong* for other kinds of items, but not needed
             },
 
diff --git a/src/librustc_metadata/schema.rs b/src/librustc_metadata/schema.rs
index 25f499bc31984..f3ff9747625f5 100644
--- a/src/librustc_metadata/schema.rs
+++ b/src/librustc_metadata/schema.rs
@@ -316,6 +316,7 @@ pub enum EntryKind<'tcx> {
     AssociatedType(AssociatedContainer),
     AssociatedExistential(AssociatedContainer),
     AssociatedConst(AssociatedContainer, ConstQualif, Lazy<RenderedConst>),
+    TraitAlias(Lazy<TraitAliasData<'tcx>>),
 }
 
 impl<'a, 'gcx> HashStable<StableHashingContext<'a>> for EntryKind<'gcx> {
@@ -370,6 +371,9 @@ impl<'a, 'gcx> HashStable<StableHashingContext<'a>> for EntryKind<'gcx> {
             EntryKind::Trait(ref trait_data) => {
                 trait_data.hash_stable(hcx, hasher);
             }
+            EntryKind::TraitAlias(ref trait_alias_data) => {
+                trait_alias_data.hash_stable(hcx, hasher);
+            }
             EntryKind::Impl(ref impl_data) => {
                 impl_data.hash_stable(hcx, hasher);
             }
@@ -474,6 +478,15 @@ impl_stable_hash_for!(struct TraitData<'tcx> {
     super_predicates
 });
 
+#[derive(RustcEncodable, RustcDecodable)]
+pub struct TraitAliasData<'tcx> {
+    pub super_predicates: Lazy<ty::GenericPredicates<'tcx>>,
+}
+
+impl_stable_hash_for!(struct TraitAliasData<'tcx> {
+    super_predicates
+});
+
 #[derive(RustcEncodable, RustcDecodable)]
 pub struct ImplData<'tcx> {
     pub polarity: hir::ImplPolarity,
diff --git a/src/librustc_resolve/build_reduced_graph.rs b/src/librustc_resolve/build_reduced_graph.rs
index a452bbf0c9d54..28b80fe3aaf29 100644
--- a/src/librustc_resolve/build_reduced_graph.rs
+++ b/src/librustc_resolve/build_reduced_graph.rs
@@ -673,6 +673,9 @@ impl<'a> Resolver<'a> {
                 }
                 module.populated.set(true);
             }
+            Def::TraitAlias(..) => {
+                self.define(parent, ident, TypeNS, (def, vis, DUMMY_SP, expansion));
+            }
             Def::Struct(..) | Def::Union(..) => {
                 self.define(parent, ident, TypeNS, (def, vis, DUMMY_SP, expansion));
 
diff --git a/src/librustc_traits/lowering/mod.rs b/src/librustc_traits/lowering/mod.rs
index 5502a1d186eee..9bdef3051e589 100644
--- a/src/librustc_traits/lowering/mod.rs
+++ b/src/librustc_traits/lowering/mod.rs
@@ -158,7 +158,8 @@ crate fn program_clauses_for<'a, 'tcx>(
     def_id: DefId,
 ) -> Clauses<'tcx> {
     match tcx.def_key(def_id).disambiguated_data.data {
-        DefPathData::Trait(_) => program_clauses_for_trait(tcx, def_id),
+        DefPathData::Trait(_) |
+        DefPathData::TraitAlias(_) => program_clauses_for_trait(tcx, def_id),
         DefPathData::Impl => program_clauses_for_impl(tcx, def_id),
         DefPathData::AssocTypeInImpl(..) => program_clauses_for_associated_type_value(tcx, def_id),
         DefPathData::AssocTypeInTrait(..) => program_clauses_for_associated_type_def(tcx, def_id),
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 534279e981d62..e89506aaf99f9 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -2,7 +2,7 @@
 //! The main routine here is `ast_ty_to_ty()`; each use is parameterized by an
 //! instance of `AstConv`.
 
-use errors::{Applicability, FatalError, DiagnosticId};
+use errors::{Applicability, DiagnosticId};
 use hir::{self, GenericArg, GenericArgs};
 use hir::def::Def;
 use hir::def_id::DefId;
@@ -690,27 +690,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
     {
         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1);
 
-        let trait_def_id = self.trait_def_id(trait_ref);
         self.ast_path_to_mono_trait_ref(trait_ref.path.span,
-                                        trait_def_id,
+                                        trait_ref.trait_def_id(),
                                         self_ty,
                                         trait_ref.path.segments.last().unwrap())
     }
 
-    /// Get the `DefId` of the given trait ref. It _must_ actually be a trait.
-    fn trait_def_id(&self, trait_ref: &hir::TraitRef) -> DefId {
-        let path = &trait_ref.path;
-        match path.def {
-            Def::Trait(trait_def_id) => trait_def_id,
-            Def::TraitAlias(alias_def_id) => alias_def_id,
-            Def::Err => {
-                FatalError.raise();
-            }
-            _ => unreachable!(),
-        }
-    }
-
-    /// The given trait ref must actually be a trait.
+    /// The given trait-ref must actually be a trait.
     pub(super) fn instantiate_poly_trait_ref_inner(&self,
         trait_ref: &hir::TraitRef,
         self_ty: Ty<'tcx>,
@@ -718,7 +704,7 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
         speculative: bool)
         -> (ty::PolyTraitRef<'tcx>, Option<Vec<Span>>)
     {
-        let trait_def_id = self.trait_def_id(trait_ref);
+        let trait_def_id = trait_ref.trait_def_id();
 
         debug!("instantiate_poly_trait_ref({:?}, def_id={:?})", trait_ref, trait_def_id);
 
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index 3a546f84469e6..93f14a2ea6f64 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -715,7 +715,7 @@ fn super_predicates_of<'a, 'tcx>(
     // In the case of trait aliases, however, we include all bounds in the where clause,
     // so e.g., `trait Foo = where u32: PartialEq<Self>` would include `u32: PartialEq<Self>`
     // as one of its "superpredicates".
-    let is_trait_alias = ty::is_trait_alias(tcx, trait_def_id);
+    let is_trait_alias = tcx.is_trait_alias(trait_def_id);
     let superbounds2 = icx.type_parameter_bounds_in_generics(
         generics, item.id, self_param_ty, OnlySelfBounds(!is_trait_alias));
 
diff --git a/src/test/ui/issues/issue-56488.rs b/src/test/ui/issues/issue-56488.rs
new file mode 100644
index 0000000000000..e2f3996927b7f
--- /dev/null
+++ b/src/test/ui/issues/issue-56488.rs
@@ -0,0 +1,13 @@
+// run-pass
+
+#![feature(trait_alias)]
+
+mod alpha {
+    pub trait A {}
+    pub trait C = A;
+}
+
+#[allow(unused_imports)]
+use alpha::C;
+
+fn main() {}
diff --git a/src/test/ui/lint/lint-incoherent-auto-trait-objects.rs b/src/test/ui/lint/lint-incoherent-auto-trait-objects.rs
new file mode 100644
index 0000000000000..0d18965ee7338
--- /dev/null
+++ b/src/test/ui/lint/lint-incoherent-auto-trait-objects.rs
@@ -0,0 +1,21 @@
+// ignore-tidy-linelength
+
+trait Foo {}
+
+impl Foo for dyn Send {}
+
+impl Foo for dyn Send + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+impl Foo for dyn Send + Sync {}
+
+impl Foo for dyn Sync + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+impl Foo for dyn Send + Sync + Send {}
+//~^ ERROR conflicting implementations
+//~| hard error
+
+fn main() {}
diff --git a/src/test/ui/lint/lint-incoherent-auto-trait-objects.stderr b/src/test/ui/lint/lint-incoherent-auto-trait-objects.stderr
new file mode 100644
index 0000000000000..928c92ef91655
--- /dev/null
+++ b/src/test/ui/lint/lint-incoherent-auto-trait-objects.stderr
@@ -0,0 +1,39 @@
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:7:1
+   |
+LL | impl Foo for dyn Send {}
+   | --------------------- first implementation here
+LL | 
+LL | impl Foo for dyn Send + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + 'static)`
+   |
+   = note: #[deny(order_dependent_trait_objects)] on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + std::marker::Sync + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:13:1
+   |
+LL | impl Foo for dyn Send + Sync {}
+   | ---------------------------- first implementation here
+LL | 
+LL | impl Foo for dyn Sync + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + std::marker::Sync + 'static)`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: conflicting implementations of trait `Foo` for type `(dyn std::marker::Send + std::marker::Sync + 'static)`: (E0119)
+  --> $DIR/lint-incoherent-auto-trait-objects.rs:17:1
+   |
+LL | impl Foo for dyn Sync + Send {}
+   | ---------------------------- first implementation here
+...
+LL | impl Foo for dyn Send + Sync + Send {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `(dyn std::marker::Send + std::marker::Sync + 'static)`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #56484 <https://github.com/rust-lang/rust/issues/56484>
+
+error: aborting due to 3 previous errors
+
diff --git a/src/test/ui/traits/auxiliary/trait_alias.rs b/src/test/ui/traits/auxiliary/trait_alias.rs
new file mode 100644
index 0000000000000..9e56b87e08813
--- /dev/null
+++ b/src/test/ui/traits/auxiliary/trait_alias.rs
@@ -0,0 +1,3 @@
+#![feature(trait_alias)]
+
+pub trait SendSync = Send + Sync;
diff --git a/src/test/ui/traits/trait-alias-cross-crate.rs b/src/test/ui/traits/trait-alias-cross-crate.rs
new file mode 100644
index 0000000000000..259fc4fa5d1ce
--- /dev/null
+++ b/src/test/ui/traits/trait-alias-cross-crate.rs
@@ -0,0 +1,17 @@
+// aux-build:trait_alias.rs
+
+#![feature(trait_alias)]
+
+extern crate trait_alias;
+
+use std::rc::Rc;
+use trait_alias::SendSync;
+
+fn use_alias<T: SendSync>() {}
+
+fn main() {
+    use_alias::<u32>();
+    use_alias::<Rc<u32>>();
+    //~^ ERROR `std::rc::Rc<u32>` cannot be sent between threads safely [E0277]
+    //~^^ ERROR `std::rc::Rc<u32>` cannot be shared between threads safely [E0277]
+}
diff --git a/src/test/ui/traits/trait-alias-cross-crate.stderr b/src/test/ui/traits/trait-alias-cross-crate.stderr
new file mode 100644
index 0000000000000..972d213ac8f8f
--- /dev/null
+++ b/src/test/ui/traits/trait-alias-cross-crate.stderr
@@ -0,0 +1,29 @@
+error[E0277]: `std::rc::Rc<u32>` cannot be sent between threads safely
+  --> $DIR/trait-alias-cross-crate.rs:14:5
+   |
+LL |     use_alias::<Rc<u32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^ `std::rc::Rc<u32>` cannot be sent between threads safely
+   |
+   = help: the trait `std::marker::Send` is not implemented for `std::rc::Rc<u32>`
+note: required by `use_alias`
+  --> $DIR/trait-alias-cross-crate.rs:10:1
+   |
+LL | fn use_alias<T: SendSync>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: `std::rc::Rc<u32>` cannot be shared between threads safely
+  --> $DIR/trait-alias-cross-crate.rs:14:5
+   |
+LL |     use_alias::<Rc<u32>>();
+   |     ^^^^^^^^^^^^^^^^^^^^ `std::rc::Rc<u32>` cannot be shared between threads safely
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::rc::Rc<u32>`
+note: required by `use_alias`
+  --> $DIR/trait-alias-cross-crate.rs:10:1
+   |
+LL | fn use_alias<T: SendSync>() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.