When a or b is an error, relate ok

Author: jackh726

chalk-solve/src/infer/unify.rs

@@ -152,6 +152,8 @@ impl<'t, I: Interner> Unifier<'t, I> {
                 }
             }
 
+            (TyKind::Error, _) | (_, TyKind::Error) => Ok(()),
+
             // Unifying `forall<X> { T }` with some other forall type `forall<X> { U }`
             (&TyKind::Function(ref fn1), &TyKind::Function(ref fn2)) => {
                 if fn1.sig == fn2.sig {
@@ -349,7 +351,6 @@ impl<'t, I: Interner> Unifier<'t, I> {
             (TyKind::Foreign(id_a), TyKind::Foreign(id_b)) => {
                 Zip::zip_with(self, variance, id_a, id_b)
             }
-            (TyKind::Error, TyKind::Error) => Ok(()),
 
             (_, _) => Err(NoSolution),
         }