Verify first-class callables can be used as parameter defaults

See https://wiki.php.net/rfc/closures_in_const_expr#future_scope

src/test/php/lang/ast/unittest/emit/ParameterTest.class.php

@@ -135,6 +135,12 @@ public function closure_as_default() {
     Assert::equals(2, $function(1));
   }
 
+  #[Test]
+  public function first_class_callable_as_default() {
+    $function= $this->param('$op= strlen(...)')->default();
+    Assert::equals(4, $function('Test'));
+  }
+
   #[Test]
   public function trailing_comma_allowed() {
     $p= $this->declare('class %T { public function fixture($param, ) { } }')