Fix lowering of resolved const inference variables

compiler/rustc_infer/src/infer/context.rs

@@ -521,7 +521,10 @@ impl<'a, 'tcx> ty::TypeFolder<TyCtxt<'tcx>> for LowerUniverseFolder<'a, 'tcx> {
         match c.kind() {
             ty::ConstKind::Infer(ty::InferConst::Var(vid)) => {
                 let vid = self.infcx.root_const_var(vid);
-                let universe = self.infcx.try_resolve_const_var(vid).unwrap_err();
+                let universe = match self.infcx.try_resolve_const_var(vid) {
+                    Ok(value) => return value.fold_with(self),
+                    Err(universe) => universe,
+                };
                 if self.for_universe.can_name(universe) {
                     c
                 } else {

compiler/rustc_next_trait_solver/src/canonical/mod.rs

@@ -392,12 +392,10 @@ where
         }
 
         let infcx = self.infcx;
-        // FIXME: make this a debug_assert.
-        // Currently proof tree evaluation can unify infer vars in original
-        // vars while not resolving them.
-        // See `tests/ui/traits/next-solver/transmute-from-async-closure.rs`
+        // Proof tree evaluation can unify inference variables in the original
+        // values without eagerly resolving them.
         let a = infcx.shallow_resolve_const(a);
-        debug_assert_eq!(b, infcx.shallow_resolve_const(b));
+        let b = infcx.shallow_resolve_const(b);
         match (a.kind(), b.kind()) {
             (
                 ty::ConstKind::Infer(ty::InferConst::Var(a_vid)),

tests/ui/traits/next-solver/lower-universe-resolved-const-issue-159703.rs

@@ -0,0 +1,19 @@
+//@ compile-flags: -Znext-solver=globally
+//@ edition: 2015
+
+#![allow(bare_trait_objects)]
+
+trait Foo {}
+
+struct BarType<const N: usize>;
+
+impl<const N: usize> Foo for BarType {}
+//~^ ERROR missing generics for struct `BarType`
+
+fn a(x: &Foo) {
+    let bar = BarType;
+    a(bar);
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/traits/next-solver/lower-universe-resolved-const-issue-159703.stderr

@@ -0,0 +1,40 @@
+error[E0107]: missing generics for struct `BarType`
+  --> $DIR/lower-universe-resolved-const-issue-159703.rs:10:30
+   |
+LL | impl<const N: usize> Foo for BarType {}
+   |                              ^^^^^^^ expected 1 generic argument
+   |
+note: struct defined here, with 1 generic parameter: `N`
+  --> $DIR/lower-universe-resolved-const-issue-159703.rs:8:8
+   |
+LL | struct BarType<const N: usize>;
+   |        ^^^^^^^ --------------
+help: add missing generic argument
+   |
+LL | impl<const N: usize> Foo for BarType<N> {}
+   |                                     +++
+
+error[E0308]: mismatched types
+  --> $DIR/lower-universe-resolved-const-issue-159703.rs:15:7
+   |
+LL |     a(bar);
+   |     - ^^^ expected `&dyn Foo`, found `BarType<_>`
+   |     |
+   |     arguments to this function are incorrect
+   |
+   = note: expected reference `&dyn Foo`
+                 found struct `BarType<_>`
+note: function defined here
+  --> $DIR/lower-universe-resolved-const-issue-159703.rs:13:4
+   |
+LL | fn a(x: &Foo) {
+   |    ^ -------
+help: consider borrowing here
+   |
+LL |     a(&bar);
+   |       +
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0107, E0308.
+For more information about an error, try `rustc --explain E0107`.