fix: return defined false from stat/lstat in scalar context

FileCheck.xs

@@ -447,8 +447,16 @@ PP(pp_overload_stat) { /* stat & lstat */
        * In such a case, we set the statcache, but do not call
        * the real op (CALL_REAL_OP)
       */
-      if ( size < 0 )
+      if ( size < 0 ) {
+        /* Match Perl's real pp_stat: in scalar/void context, push a
+         * defined false value so stat($f) returns !!0 rather than
+         * leaving the stack short (which yields undef).  In list
+         * context the empty stack is correct (empty list). */
+        if (GIMME_V != G_ARRAY) {
+          PUSHs(&PL_sv_no);
+        }
         RETURN;
+      }
 
       PUSHs( MUTABLE_SV( PL_defgv ) ); /* add *_ to the stack */
 

t/stat-scalar-context.t

@@ -0,0 +1,74 @@
+#!/usr/bin/perl -w
+
+use strict;
+use warnings;
+
+use Test2::Bundle::Extended;
+use Test2::Plugin::NoWarnings;
+
+use Overload::FileCheck qw(:all);
+
+# Mocked stat returning empty array (file not found) should behave
+# identically to Perl's real stat in scalar context: return a defined
+# false value, not undef.  A stack imbalance in the XS failure path
+# previously caused scalar stat to return undef or a stale stack value.
+
+mock_stat(sub {
+    my ($opname, $file) = @_;
+    return []                          if $file eq '/missing';
+    return stat_as_file(size => 1024)  if $file eq '/present';
+    return FALLBACK_TO_REAL_OP;
+});
+
+# --- Scalar context: missing file ---
+
+{
+    my $result = stat('/missing');
+    ok( defined($result), 'scalar stat on missing mocked file returns a defined value' );
+    ok( !$result, 'scalar stat on missing mocked file is false' );
+}
+
+# --- Boolean context: missing file ---
+
+{
+    if ( stat('/missing') ) {
+        fail('stat(/missing) should be falsy');
+    }
+    else {
+        pass('stat(/missing) is falsy in boolean context');
+    }
+}
+
+# --- Scalar context: existing file ---
+
+{
+    my $result = stat('/present');
+    ok( $result, 'scalar stat on existing mocked file is truthy' );
+}
+
+# --- List context: missing file (regression) ---
+
+{
+    my @r = stat('/missing');
+    is( scalar @r, 0, 'list stat on missing mocked file returns empty list' );
+}
+
+# --- List context: existing file (regression) ---
+
+{
+    my @r = stat('/present');
+    is( scalar @r, 13, 'list stat on existing mocked file returns 13 elements' );
+}
+
+# --- Stack integrity: scalar stat failure must not corrupt surrounding values ---
+
+{
+    my $before = 'sentinel';
+    my $s      = stat('/missing');
+    my $after  = 'sentinel';
+    is( $before, 'sentinel', 'stack not corrupted before stat call' );
+    is( $after,  'sentinel', 'stack not corrupted after stat call' );
+}
+
+unmock_stat();
+done_testing;