fix: preserve modulus semantics under no overloading

docs/about/changelog.md

@@ -9,6 +9,9 @@ priorities and future plans.
 - Restore Perl smartmatch dispatch for arrays, hashes, regexes, predicates,
   tied hashes, overloaded objects, and both execution backends.
 
+- Preserve Perl's divisor-sign modulus semantics in dynamically compiled
+  methods under `no overloading`.
+
 - Restore file-test error, stat-cache, glob-reference, and `tell` bareword
   behavior while preserving `${^LAST_FH}` for ordinary scalar arguments.
 

src/main/java/org/perlonjava/runtime/operators/MathOperators.java

@@ -1256,6 +1256,15 @@ public static RuntimeScalar integerModulus(RuntimeScalar arg1, RuntimeScalar arg
         return new RuntimeScalar(result);
     }
 
+    /** Integer modulus with Perl's divisor-sign result rule. */
+    private static RuntimeScalar modulusFromLongs(long dividend, long divisor) {
+        long result = dividend % divisor;
+        if (result != 0 && ((divisor > 0 && result < 0) || (divisor < 0 && result > 0))) {
+            result += divisor;
+        }
+        return getScalarInt(result);
+    }
+
     /** Modulus when at least one operand is already a DOUBLE (see {@link #modulus}). */
     private static RuntimeScalar modulusFromDoubles(double dividend, double divisor) {
         if (divisor == 0.0) {
@@ -1686,8 +1695,10 @@ private static RuntimeScalar arith(RuntimeScalar a, RuntimeScalar b, int op) {
                 case 3 -> y != 0 && x % y == 0
                         ? getScalarInt(x / y)
                         : new RuntimeScalar((double) x / (double) y);
-                case 4 -> y != 0 ? getScalarInt(x % y)
-                        : new RuntimeScalar((double) x % (double) y);
+                // Perl's modulus result has the divisor's sign.  Do not use
+                // Java's raw remainder here: this no-overload path is used by
+                // dynamically compiled methods under `no overloading` too.
+                case 4 -> modulusFromLongs(x, y);
                 case 5 -> new RuntimeScalar(Math.pow(x, y));
                 default -> throw new IllegalStateException();
             };
@@ -1697,7 +1708,7 @@ private static RuntimeScalar arith(RuntimeScalar a, RuntimeScalar b, int op) {
                 case 1 -> integerResult(BigInteger.valueOf(x).subtract(BigInteger.valueOf(y)));
                 case 2 -> integerResult(BigInteger.valueOf(x).multiply(BigInteger.valueOf(y)));
                 case 3 -> new RuntimeScalar((double) x / (double) y);
-                case 4 -> new RuntimeScalar((double) x % (double) y);
+                case 4 -> modulusFromDoubles((double) x, (double) y);
                 case 5 -> new RuntimeScalar(Math.pow((double) x, (double) y));
                 default -> throw new IllegalStateException();
             };

src/test/resources/unit/issue_1311_autoload_modulus.t

@@ -0,0 +1,45 @@
+use strict;
+use warnings;
+use Test::More tests => 2;
+
+{
+    package Issue1311AutoLoader;
+
+    sub install_autoloaded_method {
+        my $code = q{
+            sub local_day_of_week {
+                my $self = shift;
+                return 1 + (($self->day_of_week - $self->{locale}->first_day_of_week) % 7);
+            }
+        };
+        no overloading;
+        eval $code;
+        die $@ if $@;
+    }
+
+    sub new {
+        my ($class, $day_of_week, $first_day_of_week) = @_;
+        return bless {
+            day_of_week => $day_of_week,
+            locale => bless({ first_day_of_week => $first_day_of_week }, 'Issue1311AutoLoader::Locale'),
+        }, $class;
+    }
+
+    sub day_of_week {
+        return $_[0]->{day_of_week};
+    }
+
+    package Issue1311AutoLoader::Locale;
+
+    sub first_day_of_week {
+        return $_[0]->{first_day_of_week};
+    }
+}
+
+my $date = Issue1311AutoLoader->new(2, 7);
+Issue1311AutoLoader::install_autoloaded_method();
+
+is($date->day_of_week - $date->{locale}->first_day_of_week, -5,
+   'fixture has a negative weekday offset');
+is($date->local_day_of_week, 3,
+   'method compiled by AUTOLOAD under no overloading uses Perl modulus semantics');