Issue #24400: Introduce a distinct type for 'async def' coroutines.

Summary of changes:

1. Coroutines now have a distinct, separate from generators
   type at the C level: PyGen_Type, and a new typedef PyCoroObject.
   PyCoroObject shares the initial segment of struct layout with
   PyGenObject, making it possible to reuse existing generators
   machinery.  The new type is exposed as 'types.CoroutineType'.

   As a consequence of having a new type, CO_GENERATOR flag is
   no longer applied to coroutines.

2. Having a separate type for coroutines made it possible to add
   an __await__ method to the type.  Although it is not used by the
   interpreter (see details on that below), it makes coroutines
   naturally (without using __instancecheck__) conform to
   collections.abc.Coroutine and collections.abc.Awaitable ABCs.

   [The __instancecheck__ is still used for generator-based
   coroutines, as we don't want to add __await__ for generators.]

3. Add new opcode: GET_YIELD_FROM_ITER.  The opcode is needed to
   allow passing native coroutines to the YIELD_FROM opcode.

   Before this change, 'yield from o' expression was compiled to:

      (o)
      GET_ITER
      LOAD_CONST
      YIELD_FROM

   Now, we use GET_YIELD_FROM_ITER instead of GET_ITER.

   The reason for adding a new opcode is that GET_ITER is used
   in some contexts (such as 'for .. in' loops) where passing
   a coroutine object is invalid.

4. Add two new introspection functions to the inspec module:
   getcoroutinestate(c) and getcoroutinelocals(c).

5. inspect.iscoroutine(o) is updated to test if 'o' is a native
   coroutine object.  Before this commit it used abc.Coroutine,
   and it was requested to update inspect.isgenerator(o) to use
   abc.Generator; it was decided, however, that inspect functions
   should really be tailored for checking for native types.

6. sys.set_coroutine_wrapper(w) API is updated to work with only
   native coroutines.  Since types.coroutine decorator supports
   any type of callables now, it would be confusing that it does
   not work for all types of coroutines.

7. Exceptions logic in generators C implementation was updated
   to raise clearer messages for coroutines:

   Before: TypeError("generator raised StopIteration")
   After: TypeError("coroutine raised StopIteration")

2 files changed, 34 insertions, 26 deletions

diff --git a/Include/genobject.h b/Include/genobject.h
index ee0b130..4c71861 100644
--- a/Include/genobject.h
+++ b/Include/genobject.h
@@ -10,27 +10,26 @@ extern "C" {
 
 struct _frame; /* Avoid including frameobject.h */
 
+/* _PyGenObject_HEAD defines the initial segment of generator
+   and coroutine objects. */
+#define _PyGenObject_HEAD(prefix)                                           \
+    PyObject_HEAD                                                           \
+    /* Note: gi_frame can be NULL if the generator is "finished" */         \
+    struct _frame *prefix##_frame;                                          \
+    /* True if generator is being executed. */                              \
+    char prefix##_running;                                                  \
+    /* The code object backing the generator */                             \
+    PyObject *prefix##_code;                                                \
+    /* List of weak reference. */                                           \
+    PyObject *prefix##_weakreflist;                                         \
+    /* Name of the generator. */                                            \
+    PyObject *prefix##_name;                                                \
+    /* Qualified name of the generator. */                                  \
+    PyObject *prefix##_qualname;
+
 typedef struct {
-    PyObject_HEAD
     /* The gi_ prefix is intended to remind of generator-iterator. */
-
-    /* Note: gi_frame can be NULL if the generator is "finished" */
-    struct _frame *gi_frame;
-
-    /* True if generator is being executed. */
-    char gi_running;
-
-    /* The code object backing the generator */
-    PyObject *gi_code;
-
-    /* List of weak reference. */
-    PyObject *gi_weakreflist;
-
-    /* Name of the generator. */
-    PyObject *gi_name;
-
-    /* Qualified name of the generator. */
-    PyObject *gi_qualname;
+    _PyGenObject_HEAD(gi)
 } PyGenObject;
 
 PyAPI_DATA(PyTypeObject) PyGen_Type;
@@ -38,12 +37,6 @@ PyAPI_DATA(PyTypeObject) PyGen_Type;
 #define PyGen_Check(op) PyObject_TypeCheck(op, &PyGen_Type)
 #define PyGen_CheckExact(op) (Py_TYPE(op) == &PyGen_Type)
 
-#define PyGen_CheckCoroutineExact(op) (PyGen_CheckExact(op) && \
-                                       (((PyCodeObject*) \
-                                           ((PyGenObject*)op)->gi_code) \
-                                         ->co_flags & (CO_ITERABLE_COROUTINE | \
-                                                       CO_COROUTINE)))
-
 PyAPI_FUNC(PyObject *) PyGen_New(struct _frame *);
 PyAPI_FUNC(PyObject *) PyGen_NewWithQualName(struct _frame *,
     PyObject *name, PyObject *qualname);
@@ -52,7 +45,21 @@ PyAPI_FUNC(int) _PyGen_FetchStopIterationValue(PyObject **);
 PyObject *_PyGen_Send(PyGenObject *, PyObject *);
 PyAPI_FUNC(void) _PyGen_Finalize(PyObject *self);
 
-PyObject *_PyGen_GetAwaitableIter(PyObject *o);
+#ifndef Py_LIMITED_API
+typedef struct {
+    _PyGenObject_HEAD(cr)
+} PyCoroObject;
+
+PyAPI_DATA(PyTypeObject) PyCoro_Type;
+PyAPI_DATA(PyTypeObject) _PyCoroWrapper_Type;
+
+#define PyCoro_CheckExact(op) (Py_TYPE(op) == &PyCoro_Type)
+PyObject *_PyCoro_GetAwaitableIter(PyObject *o);
+PyAPI_FUNC(PyObject *) PyCoro_New(struct _frame *,
+    PyObject *name, PyObject *qualname);
+#endif
+
+#undef _PyGenObject_HEAD
 
 #ifdef __cplusplus
 }
diff --git a/Include/opcode.h b/Include/opcode.h
index ca59338..3f917fb 100644
--- a/Include/opcode.h
+++ b/Include/opcode.h
@@ -45,6 +45,7 @@ extern "C" {
 #define BINARY_OR                66
 #define INPLACE_POWER            67
 #define GET_ITER                 68
+#define GET_YIELD_FROM_ITER      69
 #define PRINT_EXPR               70
 #define LOAD_BUILD_CLASS         71
 #define YIELD_FROM               72