Emit alias-eq when equating numeric var and projection

compiler/rustc_infer/src/infer/combine.rs

@@ -119,20 +119,30 @@ pub fn super_combine_tys<R>(
                 self.unify_float_variable(!a_is_expected, v_id, v)
             }
 
+            // We don't expect `TyVar` or `Fresh*` vars at this point with lazy norm.
+            (
+                ty::Alias(AliasKind::Projection, _),
+                ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)),
+            )
+            | (
+                ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)),
+                ty::Alias(AliasKind::Projection, _),
+            ) if self.tcx.trait_solver_next() => {
+                bug!()
+            }
+
+            (_, ty::Alias(AliasKind::Projection, _)) | (ty::Alias(AliasKind::Projection, _), _)
+                if self.tcx.trait_solver_next() =>
+            {
+                relation.register_type_equate_obligation(a, b);
+                Ok(a)
+            }
+
             // All other cases of inference are errors
             (&ty::Infer(_), _) | (_, &ty::Infer(_)) => {
                 Err(TypeError::Sorts(ty::relate::expected_found(relation, a, b)))
             }
 
-            (ty::Alias(AliasKind::Projection, _), _) if self.tcx.trait_solver_next() => {
-                relation.register_type_equate_obligation(a, b);
-                Ok(b)
-            }
-            (_, ty::Alias(AliasKind::Projection, _)) if self.tcx.trait_solver_next() => {
-                relation.register_type_equate_obligation(b, a);
-                Ok(a)
-            }
-
             _ => ty::relate::super_relate_tys(relation, a, b),
         }
     }

compiler/rustc_middle/src/ty/mod.rs

@@ -995,7 +995,7 @@ pub fn to_alias_term_no_opaque(&self, tcx: TyCtxt<'tcx>) -> Option<AliasTy<'tcx>
 
     pub fn is_infer(&self) -> bool {
         match self.unpack() {
-            TermKind::Ty(ty) => ty.is_ty_or_numeric_infer(),
+            TermKind::Ty(ty) => ty.is_ty_var(),
             TermKind::Const(ct) => ct.is_ct_infer(),
         }
     }

tests/ui/traits/new-solver/int-var-alias-eq.rs

@@ -0,0 +1,18 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+// HIR typeck ends up equating `<_#0i as Add>::Output == _#0i`.
+// Want to make sure that we emit an alias-eq goal for this,
+// instead of treating it as a type error and bailing.
+
+fn test() {
+    // fallback
+    let x = 1 + 2;
+}
+
+fn test2() -> u32 {
+    // expectation from return ty
+    1 + 2
+}
+
+fn main() {}