diff --git a/src/Preprocessor.php b/src/Preprocessor.php
index 2d311e00..cfd3c7d9 100644
--- a/src/Preprocessor.php
+++ b/src/Preprocessor.php
@@ -83,6 +83,53 @@ class Preprocessor extends CompilerBase
         return $type . ' ' . $argInfo->name;
     }
 
+    /**
+     * A default value that can only be resolved at runtime (e.g. a class/global
+     * constant coming from a class that is not compiled into the binary) is emitted
+     * as a `php::constant(...)` call, which returns a `php::Variant`.
+     *
+     * Copy-initializing a typed (non-Variant) parameter such as `php::Int`,
+     * `php::Float`, `php::Bool`, `php::Str`, `php::Array` or `php::Object` from a
+     * `php::Variant` is rejected by C++ because the conversion is explicit:
+     *
+     *     php::Int type = php::constant(...);   // error C2440
+     *
+     * The function body already converts such values with `php::toInt(...)` /
+     * `php::toFloat(...)` / ... (see convertExprFromType), so we wrap the default
+     * with the very same conversion here. This keeps the declaration consistent with
+     * the body and produces compilable code:
+     *
+     *     php::Int type = php::toInt(php::constant(...));   // OK
+     *
+     * Parameters whose effective type is `php::Var` (including Stream/Box, which are
+     * mapped to `php::Var`) accept a `php::Variant` directly, so they are left alone.
+     */
+    protected function wrapScalarDefaultValue(string $type, string $defaultExpr): string
+    {
+        if (!str_starts_with($defaultExpr, 'php::constant(')) {
+            return $defaultExpr;
+        }
+        $target = $type;
+        if ($target === Type::STREAM || $target === Type::BOX) {
+            $target = Type::VAR;
+        }
+        static $converters = [
+            Type::INT => 'php::toInt',
+            Type::FLOAT => 'php::toFloat',
+            Type::BOOL => 'php::toBool',
+            Type::STR => 'php::toString',
+            Type::ARRAY => 'php::toArray',
+            Type::OBJECT => 'php::toObject',
+            Type::BIGINT => 'php::toBigInt',
+            Type::DECIMAL => 'php::toDecimal',
+            Type::BIGFLOAT => 'php::toBigFloat',
+        ];
+        if (isset($converters[$target])) {
+            return $converters[$target] . '(' . $defaultExpr . ')';
+        }
+        return $defaultExpr;
+    }
+
     public function getCppFile(string $file): string
     {
         $info = pathinfo($file);
@@ -466,7 +513,8 @@ class Preprocessor extends CompilerBase
                         $argInfo->default = 'php::newReference(' . $this->parseParamDefaultValue($param->default) . ')';
                     }
                 } else {
-                    $argInfo->default = $arrayInitPlan ? $arrayInitPlan->expr : $this->parseParamDefaultValue($param->default);
+                    $defaultExpr = $arrayInitPlan ? $arrayInitPlan->expr : $this->parseParamDefaultValue($param->default);
+                    $argInfo->default = $this->wrapScalarDefaultValue($argInfo->type, $defaultExpr);
                     $argInfo->arrayInitPlan = $arrayInitPlan;
                     $argInfo->defaultValue = $param->default;
                 }
diff --git a/tests/compiler/const/class-const-default-value-typed.phpt b/tests/compiler/const/class-const-default-value-typed.phpt
new file mode 100644
index 00000000..7ee526ed
--- /dev/null
+++ b/tests/compiler/const/class-const-default-value-typed.phpt
@@ -0,0 +1,25 @@
+--TEST--
+Typed parameter default value from an unresolvable (external) class constant
+--FILE--
+<?php
+
+class TypedDefault
+{
+    // \ArrayObject is an internal (external) class whose constants cannot be folded
+    // at compile time, so the default is emitted as php::constant(...). The parameter
+    // type `int` maps to php::Int, which cannot be copy-initialized from the
+    // php::Variant returned by php::constant(...). The compiler must wrap the default
+    // in php::Int(...) so the generated C++ compiles.
+    public function run(int $value = \ArrayObject::ARRAY_AS_PROPS)
+    {
+        var_dump($value);
+    }
+}
+
+function main()
+{
+    (new TypedDefault)->run();
+}
+?>
+--EXPECT--
+int(2)