gh-157451: Fix gettext.c2py precedence of unary ! before a binary operator

[winklemad]

In C the unary ! operator binds tighter than every binary operator, so !n + 1 means (!n) + 1. gettext.c2py's parser (_parse in Lib/gettext.py) emitted a bare not prefix and then appended the binary operators to the whole string, so !n + 1 compiled to not n + 1 — i.e. not (n + 1) — because Python's not binds looser than arithmetic and comparison operators.

>>> import gettext
>>> gettext.c2py('!n + 1')(0)   # was 0; C: (!0)+1 == 2
2
>>> gettext.c2py('!n < 3')(0)   # was 0; C: (!0)<3 == 1
1

This selects the wrong plural form for any .mo catalog whose Plural-Forms rule applies ! before a binary operator.

The fix negates the operand as a self-contained parenthesised unit before the binary-operator loop, so !n + 1 compiles to (not n) + 1. Double negation still normalises to 0/1 as in C (!!n → (not (not n))).

The existing test_negation only covered !!!n with no trailing binary operator — the one form that happened to work — so the precedence bug wasn't pinned. Added test_negation_precedence covering ! before +/</* and double negation.

• Fixes gettext.c2py mistranslates unary ! before a binary operator (wrong operator precedence) #157451
• ./python -m test test_gettext → SUCCESS (74 tests); the new test fails on main (AssertionError: 0 != 2) and passes with the change.
• Added a Misc/NEWS.d entry.

AI-tools disclosure (per the devguide policy): I used an AI assistant to help locate the bug and draft the patch and test. I verified the behaviour and the fix against a locally built interpreter, understand the change, and stand by it.

• Issue: gettext.c2py mistranslates unary ! before a binary operator (wrong operator precedence) #157451

[python-cla-bot]

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