[samtrion]

Problem

The latest mutation run (Stryker 5.0.0, solution mode, MTP runner) logs for four code fix providers:

An unidentified mutation in .../ThrowIfCountCodeFixProvider.cs resulted in a compile error (at 46:45) with id: CS0165, message: Use of unassigned local variable comparison
Safe Mode! Stryker will remove all mutations in RegisterCodeFixesAsync and mark them as compile error.

Same for ThrowIfLengthCodeFixProvider (46:45), ThrowIfNullOrEmptyCodeFixProvider (45:45, helperName) and ThrowIfOutOfRangeCodeFixProvider (46:55). According to the mutation report these files have 23, 23, 23 and 26 CompileError mutants (95 in total; per-file counts come from the report, not the log) with 0 Killed/Survived. Overall 439 of 1749 mutants (25.1 %) are compile errors.

Root cause

In RegisterCodeFixesAsync the guard was ifStatement is null || !TryGet...(..., out var x) || x is null. Some mutants of this combined condition break definite assignment of the out variable (CS0165), Stryker cannot recover the single mutant and removes all mutants of the method. The identified trigger is the out-var combined with the preceding ifStatement is null || operand; the proof is the before/after Stryker run, not the source reading alone.

Change

Split the guard into if (ifStatement is null) { return; } and if (!TryGet...(..., out var x) || x is null) { return; } in the four RegisterCodeFixesAsync methods. Nothing else is touched (ApplyFixAsync and the other providers are intentionally left alone; the log shows no Safe Mode for them).

Before / After

Run: https://github.com/dailydevops/arguments/actions/runs/40806141355 (mutation.yml on this branch) vs. the latest run on main (2026-10-27).

metric	before	after
CS0165 "unidentified mutation" warnings	4	0
"Safe Mode!" lines	4	0
Mutants created	1749	1753
CompileError	439 (25.1 %)	379 (21.6 %)
NoCoverage	605	653
Ignored (block already covered filter)	270	286
Tested	435	435
Killed / Survived	261 / 97	261 / 97
Mutation score	27.10 %	25.82 %
Duration	6:47	6:55

Honest reading: the Safe Mode removals and CS0165 warnings are gone and 60 fewer mutants are compile errors (less than the estimated 95). Those mutants were reclassified, not made testable: they now show up as NoCoverage (+48) and Ignored (+16), the tested set (435) and killed/survived counts are unchanged. Because NoCoverage mutants count against the score, the overall score drops from 27.10 % to 25.82 % even though the report is now more accurate. Killing these mutants requires tests that exercise RegisterCodeFixesAsync of the four providers; that is out of scope here.

Risk / what this does not change

Pure restructuring of an early-return guard; behaviour is identical. The build passes locally; the local test run could not be executed here (apphost copy error), so CI is the check.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.31%. Comparing base (b49a9f3) to head (44d8e7c).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #848      +/-   ##
==========================================
+ Coverage   90.75%   91.31%   +0.55%     
==========================================
  Files          39       39              
  Lines        2185     2187       +2     
  Branches      381      385       +4     
==========================================
+ Hits         1983     1997      +14     
+ Misses        121      113       -8     
+ Partials       81       77       -4     

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