Incorrect use of _PyLong_CompactValue()

[serhiy-storchaka]

Bug report

There are several errors related to use of _PyLong_CompactValue() in longobject.c.

• The result has type Py_ssize_t, not intptr_t. Although on most supported platforms it is the same.
• Type cast from unsigned integer to signed integer and from signed integer to unsigned integer should be explicit.
• Downcasting should be explicit.

Some of the current code may have undefined behavior.

Linked PRs

• gh-121153: Fix some errors with use of _PyLong_CompactValue() #121154
• gh-121153: Change _PyLong_CompactValue() return type to int #121536
• [3.13] gh-121153: Fix some errors with use of _PyLong_CompactValue() (GH-121154) #121900
• [3.12] gh-121153: Fix some errors with use of _PyLong_CompactValue() … #121901

[markshannon]

The result has type Py_ssize_t, not intptr_t. Although on most supported platforms it is the same.

We should change the return type to intptr_t in that case. I thought it was.