-Werror=implicit-function-declaration

Amazingly, cffi compiled just fine with some missing function
declarations.  It provided implicit definitions for them and
then casted the "int" return type up to a PyObject*.

from_cpython/Include/objimpl.h

@@ -312,13 +312,13 @@ extern PyGC_Head *_PyGC_generation0;
     (PyObject_IS_GC(obj) && \
         (!PyTuple_CheckExact(obj) || _PyObject_GC_IS_TRACKED(obj)))
 
+#endif
 PyAPI_FUNC(PyObject *) _PyObject_GC_Malloc(size_t) PYSTON_NOEXCEPT;
 PyAPI_FUNC(PyObject *) _PyObject_GC_New(PyTypeObject *) PYSTON_NOEXCEPT;
 PyAPI_FUNC(PyVarObject *) _PyObject_GC_NewVar(PyTypeObject *, Py_ssize_t) PYSTON_NOEXCEPT;
 PyAPI_FUNC(void) PyObject_GC_Track(void *) PYSTON_NOEXCEPT;
 PyAPI_FUNC(void) PyObject_GC_UnTrack(void *) PYSTON_NOEXCEPT;
 PyAPI_FUNC(void) PyObject_GC_Del(void *) PYSTON_NOEXCEPT;
-#endif
 #define PyType_IS_GC(t) ((t),1)
 #define _PyObject_GC_TRACK(o) ((void)(o))
 #define _PyObject_GC_UNTRACK(o) ((void)(o))

from_cpython/Lib/distutils/ccompiler.py

@@ -371,6 +371,7 @@ class CCompiler:
             cc_args[:0] = ['-g']
         if before:
             cc_args[:0] = before
+        cc_args = cc_args + ["-Werror=implicit-function-declaration"]
         return cc_args
 
     def _fix_compile_args(self, output_dir, macros, include_dirs):