Don't define a duplicate __init__ for dataclasses

mypy/plugins/dataclasses.py

@@ -92,7 +92,11 @@ def transform(self) -> None:
             'frozen': _get_decorator_bool_argument(self._ctx, 'frozen', False),
         }
 
-        if decorator_arguments['init']:
+        # If there are no attributes, it may be that the new semantic analyzer has not
+        # processed them yet. In order to work around this, we can simply skip generating
+        # __init__ if there are no attributes, because if the user truly did not define any,
+        # then the object default __init__ with an empty signature will be present anyway.
+        if decorator_arguments['init'] and '__init__' not in info.names and attributes:
             add_method(
                 ctx,
                 '__init__',

test-data/unit/check-dataclasses.test

@@ -16,6 +16,21 @@ Person('Jonh', 21, None)  # E: Too many arguments for "Person"
 
 [builtins fixtures/list.pyi]
 
+[case testDataclassesCustomInit]
+# flags: --python-version 3.6
+from dataclasses import dataclass
+
+@dataclass
+class A:
+    a: int
+
+    def __init__(self, a: str) -> None:
+        pass
+
+A('1')
+
+[builtins fixtures/list.pyi]
+
 [case testDataclassesBasicInheritance]
 # flags: --python-version 3.6
 from dataclasses import dataclass