dataclass_transform: use regular dataclasses.field for field_specifiers

cf https://docs.python.org/3/library/typing.html#typing.dataclass_transform:
this is needed so that type checkers properly handle attributes using
field(). Without this, any attribute using `= field(...)` is treated as
if it were a default value by mypy, which makes it impossible to use
its other arguments. This is even more problematic for
marshmallow-dataclass since it makes it impossible to pass arguments
to marshmallow using `metadata=...`

marshmallow_dataclass/__init__.py

@@ -71,8 +71,12 @@ class User:
 
 if sys.version_info >= (3, 11):
     from typing import dataclass_transform
-else:
+elif sys.version_info >= (3, 7):
     from typing_extensions import dataclass_transform
+else:
+    # @dataclass_transform() only helps us with mypy>=1.1 which is only available for python>=3.7
+    def dataclass_transform(**kwargs):
+        return lambda cls: cls
 
 
 __all__ = ["dataclass", "add_schema", "class_schema", "field_for_schema", "NewType"]
@@ -122,7 +126,7 @@ def dataclass(
 # _cls should never be specified by keyword, so start it with an
 # underscore.  The presence of _cls is used to detect if this
 # decorator is being called with parameters or not.
-@dataclass_transform()
+@dataclass_transform(field_specifiers=(dataclasses.Field, dataclasses.field))
 def dataclass(
     _cls: Optional[Type[_U]] = None,
     *,

tests/test_mypy.yml

@@ -65,3 +65,18 @@
     reveal_type(website.email)  # N: Revealed type is "builtins.str"
 
     Website(url=42, email="user@email.com")  # E: Argument "url" to "Website" has incompatible type "int"; expected "str"  [arg-type]
+- case: dataclasses_field_not_a_default
+  mypy_config: |
+    follow_imports = silent
+    plugins = marshmallow_dataclass.mypy
+    show_error_codes = true
+  env:
+    - PYTHONPATH=.
+  main: |
+    from dataclasses import field
+    from marshmallow_dataclass import dataclass
+
+    @dataclass
+    class User:
+        id: int = field(metadata={"required": True})
+        name: str