stubdoc: Fix crash on non-str docstring (#15623)

mypy/stubdoc.py

@@ -254,7 +254,7 @@ def infer_sig_from_docstring(docstr: str | None, name: str) -> list[FunctionSig]
         * docstr: docstring
         * name: name of function for which signatures are to be found
     """
-    if not docstr:
+    if not (isinstance(docstr, str) and docstr):
         return None
 
     state = DocStringParser(name)

mypy/test/teststubgen.py

@@ -1213,6 +1213,27 @@ def test(arg0: str) -> None:
         assert_equal(output, ["def test(arg0: foo.bar.Action) -> other.Thing: ..."])
         assert_equal(set(imports), {"import foo", "import other"})
 
+    def test_generate_c_function_no_crash_for_non_str_docstring(self) -> None:
+        def test(arg0: str) -> None:
+            ...
+
+        test.__doc__ = property(lambda self: "test(arg0: str) -> None")  # type: ignore[assignment]
+
+        output: list[str] = []
+        imports: list[str] = []
+        mod = ModuleType(self.__module__, "")
+        generate_c_function_stub(
+            mod,
+            "test",
+            test,
+            output=output,
+            imports=imports,
+            known_modules=[mod.__name__],
+            sig_generators=get_sig_generators(parse_options([])),
+        )
+        assert_equal(output, ["def test(*args, **kwargs) -> Any: ..."])
+        assert_equal(imports, [])
+
     def test_generate_c_property_with_pybind11(self) -> None:
         """Signatures included by PyBind11 inside property.fget are read."""