binder: make binding to Map work better with string destinations (#2554)

bind.go

@@ -131,10 +131,26 @@ func (b *DefaultBinder) bindData(destination interface{}, data map[string][]stri
 	typ := reflect.TypeOf(destination).Elem()
 	val := reflect.ValueOf(destination).Elem()
 
-	// Map
+	// Support binding to limited Map destinations:
-	if typ.Kind() == reflect.Map {
+	// - map[string][]string,
+	// - map[string]string <-- (binds first value from data slice)
+	// - map[string]interface{}
+	// You are better off binding to struct but there are user who want this map feature. Source of data for these cases are:
+	// params,query,header,form as these sources produce string values, most of the time slice of strings, actually.
+	if typ.Kind() == reflect.Map && typ.Key().Kind() == reflect.String {
+		k := typ.Elem().Kind()
+		isElemInterface := k == reflect.Interface
+		isElemString := k == reflect.String
+		isElemSliceOfStrings := k == reflect.Slice && typ.Elem().Elem().Kind() == reflect.String
+		if !(isElemSliceOfStrings || isElemString || isElemInterface) {
+			return nil
+		}
 		for k, v := range data {
-			val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))
+			if isElemString {
+				val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v[0]))
+			} else {
+				val.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(v))
+			}
 		}
 		return nil
 	}

bind_test.go

@@ -429,6 +429,62 @@ func TestBindUnsupportedMediaType(t *testing.T) {
 	testBindError(t, strings.NewReader(invalidContent), MIMEApplicationJSON, &json.SyntaxError{})
 }
 
+func TestDefaultBinder_bindDataToMap(t *testing.T) {
+	exampleData := map[string][]string{
+		"multiple": {"1", "2"},
+		"single":   {"3"},
+	}
+
+	t.Run("ok, bind to map[string]string", func(t *testing.T) {
+		dest := map[string]string{}
+		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param"))
+		assert.Equal(t,
+			map[string]string{
+				"multiple": "1",
+				"single":   "3",
+			},
+			dest,
+		)
+	})
+
+	t.Run("ok, bind to map[string][]string", func(t *testing.T) {
+		dest := map[string][]string{}
+		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param"))
+		assert.Equal(t,
+			map[string][]string{
+				"multiple": {"1", "2"},
+				"single":   {"3"},
+			},
+			dest,
+		)
+	})
+
+	t.Run("ok, bind to map[string]interface", func(t *testing.T) {
+		dest := map[string]interface{}{}
+		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param"))
+		assert.Equal(t,
+			map[string]interface{}{
+				"multiple": []string{"1", "2"},
+				"single":   []string{"3"},
+			},
+			dest,
+		)
+	})
+
+	t.Run("ok, bind to map[string]int skips", func(t *testing.T) {
+		dest := map[string]int{}
+		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param"))
+		assert.Equal(t, map[string]int{}, dest)
+	})
+
+	t.Run("ok, bind to map[string][]int skips", func(t *testing.T) {
+		dest := map[string][]int{}
+		assert.NoError(t, new(DefaultBinder).bindData(&dest, exampleData, "param"))
+		assert.Equal(t, map[string][]int{}, dest)
+	})
+
+}
+
 func TestBindbindData(t *testing.T) {
 	ts := new(bindTestStruct)
 	b := new(DefaultBinder)