fix(optimizer): fold is_null for native scalars

src/Optimizer/FuncCallOptimizer.php

@@ -693,7 +693,16 @@ trait FuncCallOptimizer
         if ($this->isNativeObjectClass($this->detectClassOfExpr($value))) {
             return '(' . $this->parseExprAsValue($value) . ' == nullptr)';
         }
-        return '(' . $this->parseExprAsValue($value) . ').isNull()';
+
+        $type = $this->detectTypeOfExpr($value);
+        $valueCode = $this->parseExprAsValue($value);
+        if ($this->isNativeType($type)) {
+            // Fixed native scalars cannot contain null. Keep evaluating the
+            // operand because a call or increment may still have side effects.
+            return '((void) (' . $valueCode . '), false)';
+        }
+
+        return '(' . $valueCode . ').isNull()';
     }
 
     protected function genIsCallable(string $n, Node\Expr\FuncCall $e, array $c): string|false

tests/compiler/type_decl/is-null-native-scalar.phpt

@@ -0,0 +1,47 @@
+--TEST--
+is_null returns false for fixed native scalars and preserves operand side effects
+--FILE--
+<?php
+
+use native_types;
+
+function checkInt(int $value): bool
+{
+    return is_null($value);
+}
+
+function checkNullableInt(?int $value): bool
+{
+    return is_null($value);
+}
+
+function emitInt(): int
+{
+    echo "emitInt\n";
+    return 42;
+}
+
+function main(): void
+{
+    $intValue = 42;
+    $floatValue = 1.5;
+    $boolValue = true;
+
+    var_dump(is_null($intValue));
+    var_dump(is_null($floatValue));
+    var_dump(is_null($boolValue));
+    var_dump(checkInt($intValue));
+    var_dump(is_null(emitInt()));
+    var_dump(checkNullableInt(null));
+    var_dump(checkNullableInt($intValue));
+}
+?>
+--EXPECT--
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+emitInt
+bool(false)
+bool(true)
+bool(false)