Support optional for custom dataclass descriptors (#15628)

Fixes: #15020

mypy/plugins/dataclasses.py

diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
index 7b3795b..c78a1b3 100644
--- a/mypy/plugins/dataclasses.py
+++ b/mypy/plugins/dataclasses.py
@@ -680,7 +680,8 @@
                     )
 
             current_attr_names.add(lhs.name)
-            init_type = self._infer_dataclass_attr_init_type(sym, lhs.name, stmt)
+            with state.strict_optional_set(self._api.options.strict_optional):
+                init_type = self._infer_dataclass_attr_init_type(sym, lhs.name, stmt)
             found_attrs[lhs.name] = DataclassAttribute(
                 name=lhs.name,
                 alias=alias,

test-data/unit/check-dataclass-transform.test

diff --git a/test-data/unit/check-dataclass-transform.test b/test-data/unit/check-dataclass-transform.test
index 53a6eea..9029582 100644
--- a/test-data/unit/check-dataclass-transform.test
+++ b/test-data/unit/check-dataclass-transform.test
@@ -1019,18 +1019,19 @@
     def __get__(self, instance: object, owner: Any) -> str: ...
     def __get__(self, instance, owner): ...
 
-    def __set__(self, instance: Any, value: bytes) -> None: ...
+    def __set__(self, instance: Any, value: bytes | None) -> None: ...
 
 @my_dataclass
 class C:
     x: Desc
 
 c = C(x=b'x')
-C(x=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "bytes"
+c = C(x=None)
+C(x=1)  # E: Argument "x" to "C" has incompatible type "int"; expected "Optional[bytes]"
 reveal_type(c.x)  # N: Revealed type is "builtins.str"
 reveal_type(C.x)  # N: Revealed type is "builtins.int"
 c.x = b'x'
-c.x = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "bytes")
+c.x = 1  # E: Incompatible types in assignment (expression has type "int", variable has type "Optional[bytes]")
 
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/dataclasses.pyi]