Add Union.from_json_object_key?

spec/std/json/serialization_spec.cr

@@ -143,6 +143,23 @@ describe "JSON serialization" do
       Hash(BigDecimal, String).from_json(%({"1234567890.123456789": "x"})).should eq({"1234567890.123456789".to_big_d => "x"})
     end
 
+    describe "Hash with union key (Union.from_json_object_key?)" do
+      it "string deprioritized" do
+        Hash(String | Int32, Nil).from_json(%({"1": null})).should eq({1 => nil})
+        Hash(String | UInt32, Nil).from_json(%({"1": null})).should eq({1 => nil})
+      end
+
+      it "string without alternative" do
+        Hash(String | Int32, Nil).from_json(%({"foo": null})).should eq({"foo" => nil})
+      end
+
+      it "no match" do
+        expect_raises JSON::ParseException, %(Can't convert "foo" into (Float64 | Int32) at line 1, column 2) do
+          Hash(Float64 | Int32, Nil).from_json(%({"foo": null}))
+        end
+      end
+    end
+
     it "raises an error Hash(String, Int32)#from_json with null value" do
       expect_raises(JSON::ParseException, "Expected Int but was Null") do
         Hash(String, Int32).from_json(%({"foo": 1, "bar": 2, "baz": null}))

src/json/from_json.cr

@@ -440,6 +440,28 @@ def Union.new(pull : JSON::PullParser)
   {% end %}
 end
 
+def Union.from_json_object_key?(key : String)
+  {% begin %}
+    # String must come last because any key can be parsed into a String.
+    # So, we give a chance first to other types in the union to be parsed.
+    {% string_type = T.find { |type| type == ::String } %}
+
+    {% for type in T %}
+      {% unless type == string_type %}
+        if result = {{ type }}.from_json_object_key?(key)
+          return result
+        end
+      {% end %}
+    {% end %}
+
+    {% if string_type %}
+      if result = {{ string_type }}.from_json_object_key?(key)
+        return result
+      end
+    {% end %}
+  {% end %}
+end
+
 # Reads a string from JSON parser as a time formatted according to [RFC 3339](https://tools.ietf.org/html/rfc3339)
 # or other variations of [ISO 8601](http://xml.coverpages.org/ISO-FDIS-8601.pdf).
 #