fix: resolve NV sentinel collision for direct mock_file_check

FileCheck.xs

@@ -562,7 +562,19 @@ BOOT:
          newCONSTSUB(stash, "CHECK_IS_TRUE",         &PL_sv_yes );   /* could use newSViv(1) or &PL_sv_yes */
          newCONSTSUB(stash, "CHECK_IS_FALSE",        &PL_sv_no );    /* could use newSViv(0) or &PL_sv_no  */
          newCONSTSUB(stash, "CHECK_IS_NULL",         &PL_sv_undef );
-         newCONSTSUB(stash, "FALLBACK_TO_REAL_OP",  newSVnv(-1) );
+         /* FALLBACK_TO_REAL_OP is a dualvar: numeric -1, string
+          * "__OFC_FALLBACK__".  The string form lets the Perl-layer
+          * _check() distinguish a real -1.0 NV result (e.g. -M on a
+          * file 1 day in the future) from the sentinel.  Numeric
+          * comparisons (== -1) keep working for backward compat. */
+         {
+             SV *fb = newSVnv(-1);
+             sv_setpv(fb, "__OFC_FALLBACK__");
+             SvNOK_on(fb);
+             SvIOK_on(fb);
+             SvIV_set(fb, -1);
+             newCONSTSUB(stash, "FALLBACK_TO_REAL_OP", fb);
+         }
 
          /* provide constants to add entry in a fake stat array */
 

lib/Overload/FileCheck.pm

@@ -511,8 +511,10 @@ sub _check {
         if ( ref $out eq 'SCALAR' ) {
             return ( CHECK_IS_TRUE, $$out );
         }
-        # Bare scalar: from direct mock_file_check callbacks
-        if ( !ref $out && $out == FALLBACK_TO_REAL_OP ) {
+        # Bare scalar: from direct mock_file_check callbacks.
+        # Use string comparison to detect the dualvar sentinel —
+        # numeric == would collide with a real -1.0 value.
+        if ( !ref $out && "$out" eq '__OFC_FALLBACK__' ) {
             return (FALLBACK_TO_REAL_OP);
         }
         return ( CHECK_IS_TRUE, $out );

t/01_boot.t

@@ -20,7 +20,7 @@ is Overload::FileCheck::_loaded(), 1, '_loaded';
 
 is int Overload::FileCheck::CHECK_IS_TRUE(),  1, "CHECK_IS_TRUE";
 is int Overload::FileCheck::CHECK_IS_FALSE(), 0, "CHECK_IS_FALSE";
-is Overload::FileCheck::FALLBACK_TO_REAL_OP(), -1, "FALLBACK_TO_REAL_OP";
+ok Overload::FileCheck::FALLBACK_TO_REAL_OP() == -1, "FALLBACK_TO_REAL_OP == -1";
 
 my @ops = qw{
   OP_FTRREAD

t/nv-sentinel-collision.t

@@ -136,4 +136,72 @@ subtest 'mock_all_from_stat: other NV values pass through correctly' => sub {
     unmock_stat();
 };
 
+subtest 'direct mock_file_check: -M returns -1.0 without sentinel collision' => sub {
+    mock_file_check( '-M' => sub {
+        my ($file) = @_;
+        return -1.0 if $file eq '/test/exactly_minus_one';
+        return FALLBACK_TO_REAL_OP;
+    });
+
+    my $age = -M '/test/exactly_minus_one';
+    ok( defined $age, '-M returns a defined value (not undef from fallback)' );
+    ok( abs($age - (-1.0)) < 0.01, "-M returns -1.0 (got: $age)" )
+        if defined $age;
+
+    my $real = -M '/nonexistent/direct/mock/fallback';
+    ok( !defined $real, 'FALLBACK_TO_REAL_OP still delegates to real OP' );
+
+    unmock_file_check('-M');
+};
+
+subtest 'direct mock_file_check: -A returns -1.0 without sentinel collision' => sub {
+    mock_file_check( '-A' => sub {
+        my ($file) = @_;
+        return -1.0 if $file eq '/test/exactly_minus_one';
+        return FALLBACK_TO_REAL_OP;
+    });
+
+    my $age = -A '/test/exactly_minus_one';
+    ok( defined $age, '-A returns a defined value' );
+    ok( abs($age - (-1.0)) < 0.01, "-A returns -1.0 (got: $age)" )
+        if defined $age;
+
+    unmock_file_check('-A');
+};
+
+subtest 'direct mock_file_check: -C returns -1.0 without sentinel collision' => sub {
+    mock_file_check( '-C' => sub {
+        my ($file) = @_;
+        return -1.0 if $file eq '/test/exactly_minus_one';
+        return FALLBACK_TO_REAL_OP;
+    });
+
+    my $age = -C '/test/exactly_minus_one';
+    ok( defined $age, '-C returns a defined value' );
+    ok( abs($age - (-1.0)) < 0.01, "-C returns -1.0 (got: $age)" )
+        if defined $age;
+
+    unmock_file_check('-C');
+};
+
+subtest 'FALLBACK_TO_REAL_OP constant backward compat' => sub {
+    ok( FALLBACK_TO_REAL_OP == -1, 'numeric comparison still works' );
+    ok( FALLBACK_TO_REAL_OP < 0,   'numeric ordering still works' );
+};
+
+subtest 'direct mock_file_check: scalar ref workaround also works' => sub {
+    mock_file_check( '-M' => sub {
+        my ($file) = @_;
+        return \(-1.0) if $file eq '/test/ref_wrap';
+        return FALLBACK_TO_REAL_OP;
+    });
+
+    my $age = -M '/test/ref_wrap';
+    ok( defined $age, '-M returns defined for scalar ref wrapped -1.0' );
+    ok( abs($age - (-1.0)) < 0.01, "-M returns -1.0 via scalar ref (got: $age)" )
+        if defined $age;
+
+    unmock_file_check('-M');
+};
+
 done_testing;