Instantiate closure synthetic substs in root universe

compiler/rustc_hir_typeck/src/closure.rs

@@ -98,7 +98,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
         );
 
-        let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
+        let tupled_upvars_ty = self.next_root_ty_var(TypeVariableOrigin {
             kind: TypeVariableOriginKind::ClosureSynthetic,
             span: self.tcx.def_span(expr_def_id),
         });
@@ -143,7 +143,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             // Create a type variable (for now) to represent the closure kind.
             // It will be unified during the upvar inference phase (`upvar.rs`)
-            None => self.next_ty_var(TypeVariableOrigin {
+            None => self.next_root_ty_var(TypeVariableOrigin {
                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
                 kind: TypeVariableOriginKind::ClosureSynthetic,
                 span: expr_span,

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -189,6 +189,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn errors_reported_since_creation(&self) -> bool {
         self.tcx.sess.err_count() > self.err_count_on_creation
     }
+
+    pub fn next_root_ty_var(&self, origin: TypeVariableOrigin) -> Ty<'tcx> {
+        self.tcx.mk_ty_var(self.next_ty_var_id_in_universe(origin, ty::UniverseIndex::ROOT))
+    }
 }
 
 impl<'a, 'tcx> Deref for FnCtxt<'a, 'tcx> {

tests/ui/traits/new-solver/closure-substs-ambiguity.rs

@@ -0,0 +1,7 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+fn main() {
+    let mut x: Vec<_> = vec![];
+    x.extend(Some(1i32).into_iter().map(|x| x));
+}