Revert "bpo-40170: PyType_HasFeature() now always calls PyType_GetFlags() (GH-19378)" (GH-21390)

This partially reverts commit 45ec5b9.
(cherry picked from commit b26a0db)

Co-authored-by: Victor Stinner <[email protected]>

Author: miss-islington

Include/object.h

@@ -618,8 +618,16 @@ times.
 
 
 static inline int
-PyType_HasFeature(PyTypeObject *type, unsigned long feature) {
-    return ((PyType_GetFlags(type) & feature) != 0);
+PyType_HasFeature(PyTypeObject *type, unsigned long feature)
+{
+    unsigned long flags;
+#ifdef Py_LIMITED_API
+    // PyTypeObject is opaque in the limited C API
+    flags = PyType_GetFlags(type);
+#else
+    flags = type->tp_flags;
+#endif
+    return ((flags & feature) != 0);
 }
 
 #define PyType_FastSubclass(type, flag) PyType_HasFeature(type, flag)

Misc/NEWS.d/next/C API/2020-07-08-10-14-52.bpo-40170.N6Qx1i.rst

@@ -0,0 +1,4 @@
+Revert :c:func:`PyType_HasFeature` change: it reads again directly the
+:c:member:`PyTypeObject.tp_flags` member when the limited C API is not used,
+rather than always calling :c:func:`PyType_GetFlags` which hides implementation
+details.