Discard matching types when restricting a union with another one

spec/compiler/type_inference/def_overload_spec.cr

@@ -543,6 +543,16 @@ describe "Type inference: def overload" do
       ") { int32.metaclass }
   end
 
+  it "gets free variable from union restriction (2)" do
+    assert_type("
+      def foo(x : Nil | U)
+        U
+      end
+
+      foo(nil || 1)
+      ") { int32.metaclass }
+  end
+
   it "gets free variable from union restriction without a union" do
     assert_type("
       def foo(x : Nil | U)

src/compiler/crystal/type_inference/restrictions.cr

@@ -203,11 +203,15 @@ module Crystal
 
     def restrict(other : Union, owner, type_lookup, free_vars)
       types = [] of Type
+      discarded = [] of Type
       other.types.each do |other_type|
         self.union_types.each do |type|
+          next if discarded.includes?(type)
+
           restricted = type.restrict(other_type, owner, type_lookup, free_vars)
           if restricted
             types << restricted
+            discarded << type
             break
           end
         end