Fix absl::monostate support against C++14

pybind11_abseil/absl_casters.h

@@ -667,6 +667,9 @@ template <typename... Ts>
 struct type_caster<absl::variant<Ts...>>
     : variant_caster<absl::variant<Ts...>> {};
 
+template <>
+struct type_caster<absl::monostate>
+    : void_caster<absl::monostate> {};
 #endif
 
 }  // namespace detail

pybind11_abseil/tests/absl_example.cc

@@ -344,6 +344,18 @@ std::vector<absl::variant<A*, B*>> Identity(
   return value;
 }
 
+bool CheckVariant(const absl::variant<absl::monostate, int> variant,
+                  bool given, int value) {
+  if (!given && !absl::holds_alternative<int>(variant)) return true;
+  if (given && absl::holds_alternative<int>(variant) &&
+      absl::get<int>(variant) == value)
+    return true;
+  return false;
+}
+
+absl::variant<absl::monostate, int> MakeVariant() { return {}; }
+absl::variant<absl::monostate, int> MakeVariant(int value) { return value; }
+
 }  // namespace test
 }  // namespace pybind11
 
@@ -487,6 +499,15 @@ PYBIND11_MODULE(absl_example, m) {
   m.def("VariantToInt", &VariantToInt);
   m.def("Identity", &Identity);
   m.def("IdentityWithCopy", &IdentityWithCopy);
+
+  m.def("check_variant", &CheckVariant,
+        arg("variant") = absl::variant<absl::monostate, int>{},
+        arg("given") = false, arg("value") = 0);
+  m.def("make_variant",
+        (absl::variant<absl::monostate, int> (*)())&MakeVariant);
+  m.def("make_variant",
+        (absl::variant<absl::monostate, int> (*)(int))&MakeVariant,
+        arg("value"));
 }
 
 }  // namespace test

pybind11_abseil/tests/absl_test.py

@@ -612,6 +612,21 @@ def test_variant(self):
       else:
         self.assertNotEqual(objs, vector)
 
+  def test_pass_default_nullopt(self):
+    self.assertTrue(absl_example.check_variant())
+
+  def test_pass_value(self):
+    self.assertTrue(absl_example.check_variant(5, True, 5))
+
+  def test_pass_none(self):
+    self.assertTrue(absl_example.check_variant(None, False))
+
+  def test_return_value(self):
+    self.assertEqual(absl_example.make_variant(5), 5)
+
+  def test_return_none(self):
+    self.assertIsNone(absl_example.make_variant())
+
 
 if __name__ == '__main__':
   absltest.main()