correctness: CHECK_IS_FALSE and CHECK_IS_NULL return identical undef

[Koan-Bot]

Summary

FT_RETURNNO is defined in FileCheck.h but never used. The XS handler pp_overload_ft_yes_no maps CHECK_IS_FALSE (0) to FT_RETURNUNDEF instead of FT_RETURNNO, making CHECK_IS_FALSE and CHECK_IS_NULL indistinguishable from the caller's perspective.

Impact

In Perl, file-test operators have two distinct "false" results:

• Defined false (''): "check failed but file is known" — e.g., -f "/tmp" (a directory)
• Undef: "file not found / unknown" — e.g., -e "/nonexistent"

Users can distinguish these with defined(). With the current code:

mock_file_check('-f', sub { CHECK_IS_FALSE });
defined(-f "something")  # returns false — WRONG, should be true

The XS comment at line 68-71 documents the intended behavior correctly:

*  0 check is false -> OP returns No
* -2 check is null  -> OP returns undef

But the implementation at line 270 doesn't match:

if ( check_status == 0 )  FT_RETURNUNDEF;  // should be FT_RETURNNO

Fix

PR #85

🤖 Created by Kōan from audit session