Added [prop-decorator] code for unsupported property decorators (#14461) (#16571)

Using a decorator before a @Property now results in the narrower
`prop-decorator` code, which is a subcode of `misc` for backward
compatibility.

I would have preferred to add a more general Unsupported error code and
have this be a subcode of that, but this has to be a subcode of misc for
backward compatibility.

Fixes #14461

Author: analog-cbarber

docs/source/error_code_list.rst

@@ -1171,6 +1171,29 @@ annotations in an unchecked function:
 Note that mypy will still exit with return code ``0``, since such behaviour is
 specified by :pep:`484`.
 
+.. _code-prop-decorator:
+
+Decorator preceding property not supported [prop-decorator]
+-----------------------------------------------------------
+
+Mypy does not yet support analysis of decorators that precede the property
+decorator. If the decorator does not preserve the declared type of the property,
+mypy will not infer the correct type for the declaration. If the decorator cannot
+be moved after the ``@property`` decorator, then you must use a type ignore
+comment:
+
+.. code-block:: python
+
+    class MyClass
+        @special  # type: ignore[prop-decorator]
+        @property
+        def magic(self) -> str:
+            return "xyzzy"
+
+.. note::
+
+    For backward compatibility, this error code is a subcode of the generic ``[misc]`` code.
+
 .. _code-syntax:
 
 Report syntax errors [syntax]

mypy/errorcodes.py

@@ -262,7 +262,6 @@ def __hash__(self) -> int:
     default_enabled=False,
 )
 
-
 # Syntax errors are often blocking.
 SYNTAX: Final[ErrorCode] = ErrorCode("syntax", "Report syntax errors", "General")
 
@@ -281,6 +280,13 @@ def __hash__(self) -> int:
     sub_code_of=MISC,
 )
 
+PROPERTY_DECORATOR = ErrorCode(
+    "prop-decorator",
+    "Decorators on top of @property are not supported",
+    "General",
+    sub_code_of=MISC,
+)
+
 NARROWED_TYPE_NOT_SUBTYPE: Final[ErrorCode] = ErrorCode(
     "narrowed-type-not-subtype",
     "Warn if a TypeIs function's narrowed type is not a subtype of the original type",

mypy/semanal.py

@@ -56,7 +56,7 @@
 
 from mypy import errorcodes as codes, message_registry
 from mypy.constant_fold import constant_fold_expr
-from mypy.errorcodes import ErrorCode
+from mypy.errorcodes import PROPERTY_DECORATOR, ErrorCode
 from mypy.errors import Errors, report_internal_error
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
 from mypy.messages import (
@@ -1620,7 +1620,9 @@ def visit_decorator(self, dec: Decorator) -> None:
         if not no_type_check and self.recurse_into_functions:
             dec.func.accept(self)
         if could_be_decorated_property and dec.decorators and dec.var.is_property:
-            self.fail("Decorators on top of @property are not supported", dec)
+            self.fail(
+                "Decorators on top of @property are not supported", dec, code=PROPERTY_DECORATOR
+            )
         if (dec.func.is_static or dec.func.is_class) and dec.var.is_property:
             self.fail("Only instance methods can be decorated with @property", dec)
         if dec.func.abstract_status == IS_ABSTRACT and dec.func.is_final:

test-data/unit/semanal-errors.test

@@ -1241,6 +1241,12 @@ class A:
     @property  # OK
     @dec
     def g(self) -> int: pass
+    @dec  # type: ignore[misc]
+    @property
+    def h(self) -> int: pass
+    @dec  # type: ignore[prop-decorator]
+    @property
+    def i(self) -> int: pass
 [builtins fixtures/property.pyi]
 [out]