Fix is_set for optional proto3 fields

src/betterproto/__init__.py

@@ -1317,7 +1317,12 @@ def is_set(self, name: str) -> bool:
         :class:`bool`
             `True` if field has been set, otherwise `False`.
         """
-        return self.__raw_get(name) is not PLACEHOLDER
+        default = (
+            PLACEHOLDER
+            if not self._betterproto.meta_by_field_name[name].optional
+            else None
+        )
+        return self.__raw_get(name) is not default
 
 
 def serialized_on_wire(message: Message) -> bool:

tests/test_features.py

@@ -517,3 +517,15 @@ class Spam(betterproto.Message):
     assert spam == deepcopied
     assert spam is not deepcopied
     assert spam.baz is not deepcopied.baz
+
+
+def test_is_set():
+    @dataclass
+    class Spam(betterproto.Message):
+        foo: bool = betterproto.bool_field(1)
+        bar: Optional[int] = betterproto.int32_field(2, optional=True)
+
+    assert not Spam().is_set("foo")
+    assert not Spam().is_set("bar")
+    assert Spam(foo=True).is_set("foo")
+    assert Spam(foo=True, bar=0).is_set("bar")