gh-155992: Fix missing LINE event for first line of a code object

[PrathamGhaywat]

Fixes #155992.

The sys.monitoring LINE event did not fire for the first line of a
code object when the def/lambda body shared that line. Since 3.13,
RESUME is specialized to RESUME_CHECK, so the special case in
_Py_call_instrumentation_line() that forces the first LINE event
(when the previous instruction is RESUME/INSTRUMENTED_RESUME) no
longer matched. The check now uses _PyOpcode_Deopt, covering all
RESUME specializations.

Tests were added to LineMonitoringTest in Lib/test/test_monitoring.py
for single-line def bodies, multi-line bodies starting on the def
line, and lambda. All of test_monitoring, test_trace,
test_profile, and test_sys_settrace pass.

• Issue: sys.monitoring in 3.13+ does not trigger LINE event when there is no line break before #155992

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[Copilot]

Pull request overview

Fixes a regression in CPython’s sys.monitoring LINE event emission where the first line of a code object could be skipped when the function/lambda body starts on the same physical line as def/lambda. The core fix updates the instrumentation logic to recognize specialized RESUME opcodes by comparing against _PyOpcode_Deopt, and adds regression tests plus a NEWS entry.

Changes:

• Update _Py_call_instrumentation_line() to treat all RESUME specializations as RESUME via _PyOpcode_Deopt.
• Add LineMonitoringTest cases covering single-line def, multi-line bodies starting on the def line, and lambda.
• Add a Core/Builtins NEWS entry documenting the regression fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
Python/instrumentation.c	Adjusts the “force first LINE event” special-case to account for specialized RESUME opcodes via deopt mapping.
Lib/test/test_monitoring.py	Adds regression tests ensuring LINE events include the first line for def/lambda bodies sharing the header line.
Misc/NEWS.d/next/Core_and_Builtins/2026-08-18-12-01-47.gh-issue-155992.Yl4gI2.rst	Documents the sys.monitoring LINE-event regression fix in the NEWS stream.

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