Description:
------------
Hello,

a fatal error (no catchable) is thrown when I attempt to call the mehod getReturnType from a class inheriting of ReflectionFunctionAbstract

Do you think it's a normal behavior ?

Test script:
---------------
<?php

class MockReflectionMethod extends ReflectionFunctionAbstract
{
    public static function export() {}
    public function __toString() {}
}


try {
    (new MockReflectionMethod)->isStatic();    
} catch(Throwable $e) {
    echo $e->getMessage();
}

try {
    (new MockReflectionMethod)->getReturnType();
} catch(Throwable $e) {
    echo $e->getMessage();
}

// output:
// Call to undefined method MockReflectionMethod::isStatic()
// Fatal error: ReflectionFunctionAbstract::getReturnType(): Internal error: Failed to retrieve the reflection object

Actual result:
--------------
http://3v4l.org/7LjRK