fix: bind Date and Time query params as scalar values (#21)

BindQueryParameter treated all structs as key-value objects in the
non-exploded form path, causing types.Date and time.Time to fail with
"property/values need to be pairs". Scalar struct types that implement
Binder or encoding.TextUnmarshaler are now bound directly via their
interface methods instead of being routed to bindSplitPartsToDestinationStruct.

Also adds Binder implementation to types.Date so it self-identifies as a
scalar binding target.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

bindparam.go

@@ -444,7 +444,24 @@ func BindQueryParameter(style string, explode bool, required bool, paramName str
 		case reflect.Slice:
 			err = bindSplitPartsToDestinationArray(parts, output)
 		case reflect.Struct:
-			err = bindSplitPartsToDestinationStruct(paramName, parts, explode, output)
+			// Some struct types (e.g. types.Date, time.Time) are scalar values
+			// that should be bound from a single string, not decomposed as
+			// key-value objects. Detect these via the Binder and
+			// TextUnmarshaler interfaces.
+			switch v := output.(type) {
+			case Binder:
+				if len(parts) != 1 {
+					return fmt.Errorf("multiple values for single value parameter '%s'", paramName)
+				}
+				err = v.Bind(parts[0])
+			case encoding.TextUnmarshaler:
+				if len(parts) != 1 {
+					return fmt.Errorf("multiple values for single value parameter '%s'", paramName)
+				}
+				err = v.UnmarshalText([]byte(parts[0]))
+			default:
+				err = bindSplitPartsToDestinationStruct(paramName, parts, explode, output)
+			}
 		default:
 			if len(parts) == 0 {
 				if required {

bindparam_test.go

@@ -336,6 +336,107 @@ func TestBindQueryParameter(t *testing.T) {
 		assert.Equal(t, expected, birthday)
 	})
 
+	// Regression tests for https://github.com/oapi-codegen/runtime/issues/21
+	// types.Date should bind correctly as a query parameter in all configurations.
+	t.Run("date_form_explode_required", func(t *testing.T) {
+		expectedDate := types.Date{Time: time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)}
+		var date types.Date
+		queryParams := url.Values{
+			"date": {"2023-01-01"},
+		}
+		err := BindQueryParameter("form", true, true, "date", queryParams, &date)
+		assert.NoError(t, err)
+		assert.Equal(t, expectedDate, date)
+	})
+
+	t.Run("date_form_explode_optional", func(t *testing.T) {
+		expectedDate := types.Date{Time: time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)}
+		var date *types.Date
+		queryParams := url.Values{
+			"date": {"2023-01-01"},
+		}
+		err := BindQueryParameter("form", true, false, "date", queryParams, &date)
+		assert.NoError(t, err)
+		require.NotNil(t, date)
+		assert.Equal(t, expectedDate, *date)
+	})
+
+	t.Run("date_form_explode_optional_missing", func(t *testing.T) {
+		var date *types.Date
+		queryParams := url.Values{}
+		err := BindQueryParameter("form", true, false, "date", queryParams, &date)
+		assert.NoError(t, err)
+		assert.Nil(t, date)
+	})
+
+	t.Run("date_form_no_explode_required", func(t *testing.T) {
+		expectedDate := types.Date{Time: time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)}
+		var date types.Date
+		queryParams := url.Values{
+			"date": {"2023-01-01"},
+		}
+		err := BindQueryParameter("form", false, true, "date", queryParams, &date)
+		assert.NoError(t, err)
+		assert.Equal(t, expectedDate, date)
+	})
+
+	t.Run("date_form_no_explode_optional", func(t *testing.T) {
+		expectedDate := types.Date{Time: time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)}
+		var date *types.Date
+		queryParams := url.Values{
+			"date": {"2023-01-01"},
+		}
+		err := BindQueryParameter("form", false, false, "date", queryParams, &date)
+		assert.NoError(t, err)
+		require.NotNil(t, date)
+		assert.Equal(t, expectedDate, *date)
+	})
+
+	// time.Time has the same bug as types.Date for form/no-explode.
+	t.Run("time_form_no_explode_required", func(t *testing.T) {
+		expectedTime := time.Date(2020, 12, 9, 16, 9, 53, 0, time.UTC)
+		var ts time.Time
+		queryParams := url.Values{
+			"ts": {"2020-12-09T16:09:53Z"},
+		}
+		err := BindQueryParameter("form", false, true, "ts", queryParams, &ts)
+		assert.NoError(t, err)
+		assert.Equal(t, expectedTime, ts)
+	})
+
+	t.Run("date_in_struct_form_explode", func(t *testing.T) {
+		type Params struct {
+			Name      string     `json:"name"`
+			StartDate types.Date `json:"start_date"`
+		}
+		queryParams := url.Values{
+			"name":       {"test"},
+			"start_date": {"2023-06-15"},
+		}
+		var params Params
+		err := BindQueryParameter("form", true, true, "params", queryParams, &params)
+		assert.NoError(t, err)
+		assert.Equal(t, "test", params.Name)
+		assert.Equal(t, types.Date{Time: time.Date(2023, 6, 15, 0, 0, 0, 0, time.UTC)}, params.StartDate)
+	})
+
+	t.Run("date_pointer_in_struct_form_explode", func(t *testing.T) {
+		type Params struct {
+			Name      string      `json:"name"`
+			StartDate *types.Date `json:"start_date"`
+		}
+		queryParams := url.Values{
+			"name":       {"test"},
+			"start_date": {"2023-06-15"},
+		}
+		var params Params
+		err := BindQueryParameter("form", true, true, "params", queryParams, &params)
+		assert.NoError(t, err)
+		assert.Equal(t, "test", params.Name)
+		require.NotNil(t, params.StartDate)
+		assert.Equal(t, types.Date{Time: time.Date(2023, 6, 15, 0, 0, 0, 0, time.UTC)}, *params.StartDate)
+	})
+
 	t.Run("optional", func(t *testing.T) {
 		queryParams := url.Values{
 			"time":   {"2020-12-09T16:09:53+00:00"},

types/date.go

@@ -41,3 +41,18 @@ func (d *Date) UnmarshalText(data []byte) error {
 	d.Time = parsed
 	return nil
 }
+
+// Bind implements the runtime.Binder interface so that Date is treated as a
+// scalar value when binding query parameters rather than being decomposed as
+// a struct with key-value pairs.
+func (d *Date) Bind(src string) error {
+	if src == "" {
+		return nil
+	}
+	parsed, err := time.Parse(DateFormat, src)
+	if err != nil {
+		return err
+	}
+	d.Time = parsed
+	return nil
+}