More nits

compiler/rustc_middle/src/ty/mod.rs

@@ -2506,6 +2506,10 @@ impl<'tcx> TyCtxt<'tcx> {
         self.trait_def(trait_def_id).has_auto_impl
     }
 
+    pub fn trait_is_coinductive(self, trait_def_id: DefId) -> bool {
+        self.trait_is_auto(trait_def_id) || self.lang_items().sized_trait() == Some(trait_def_id)
+    }
+
     /// Returns layout of a generator. Layout might be unavailable if the
     /// generator is tainted by errors.
     pub fn generator_layout(self, def_id: DefId) -> Option<&'tcx GeneratorLayout<'tcx>> {

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -959,10 +959,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
     fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
         let result = match predicate.kind().skip_binder() {
-            ty::PredicateKind::Trait(ref data) => {
-                self.tcx().trait_is_auto(data.def_id())
-                    || self.tcx().lang_items().sized_trait() == Some(data.def_id())
-            }
+            ty::PredicateKind::Trait(ref data) => self.tcx().trait_is_coinductive(data.def_id()),
             ty::PredicateKind::WellFormed(_) => true,
             _ => false,
         };

src/test/ui/sized/coinductive-1-gat.rs

@@ -0,0 +1,14 @@
+// check-pass
+struct Node<C: Trait>(C::Assoc::<Self>);
+
+trait Trait {
+    type Assoc<T>;
+}
+
+impl Trait for Vec<()> {
+    type Assoc<T> = Vec<T>;
+}
+
+fn main() {
+    let _ = Node::<Vec<()>>(Vec::new());
+}