Incorrect warning when assign an unsigned integer member

[serhiy-storchaka]

Assigning a negative value to an unsigned integer member can emit a RuntimeWarning.

$ ./python -Wa
>>> import _testcapi
>>> ts = _testcapi._test_structmembersType_NewAPI()
>>> ts.T_UBYTE = -42
<stdin>:1: RuntimeWarning: Truncation of value to unsigned char
>>> ts.T_USHORT = -42
<stdin>:1: RuntimeWarning: Truncation of value to unsigned short
>>> ts.T_UINT = -42
<stdin>:1: RuntimeWarning: Writing negative value into unsigned field
<stdin>:1: RuntimeWarning: Truncation of value to unsigned int
>>> ts.T_ULONG = -42
<stdin>:1: RuntimeWarning: Writing negative value into unsigned field

(And for T_ULONGLONG it raises an OverflowError, but this is perhaps another issue).

The first issue is that it emits two warnings for T_UINT. It is also weird that warnings are different.

The second issue is that if the value is not an int subclass, it emits a warning also for positive values for T_UINT and T_ULONG:

>>> class Index:
...     def __init__(self, value):
...         self.value = value
...     def __index__(self):
...         return self.value
... 
>>> ts.T_UBYTE = Index(42)
>>> ts.T_USHORT = Index(42)
>>> ts.T_UINT = Index(42)
<stdin>:1: RuntimeWarning: Writing negative value into unsigned field
>>> ts.T_ULONG = Index(42)
<stdin>:1: RuntimeWarning: Writing negative value into unsigned field

(No warning is emitted for assigning a negative index to T_ULONGLONG, but this is perhaps another issue).

Linked PRs

• gh-114388: Fix warnings when assign an unsigned integer member #114391
• gh-114388: Fix an error in GH-114391 #115000
• [3.12] gh-114388: Fix warnings when assign an unsigned integer member (GH-114391) #115001
• [3.11] gh-114388: Fix warnings when assign an unsigned integer member (GH-114391) #115002

[encukou]

With this change, there's a new warning emitted for “Writing negative value into unsigned field”.
@zooba, you might be interested in this