How can constructor definitions from traits collide?

Question

In PHP 5.4.9, the following example triggers the fatal error "B has colliding constructor definitions coming from traits".

trait T {
    public function __construct () {
        echo __CLASS__ . ": constructor called.\n";
    }
}

class A {
    use T;
}

class B extends A {
    use T;
}

There's no problem when the trait contains a different method than the constructor, and no problem when the constructor is actually copied into the classes (without using traits, the "language-assisted copy & paste" feature).

What's so special about the constructor here? Shouldn't PHP be able to figure out that one of them overrides the other? I couldn't find anything about this limitation in the manual.

This related question mentions a way to get around the problem (by using aliases for trait methods), but not what's causing it in the first place.

Answer 1

Try what happens with the following code:

class A {
    use T;
    use T;
}

Because this is what you effectively wrote by extending from A and then using T again for B.

If you need to use trait T in base and subclasses, use it only in the base-class.

If you need it in subclasses only, use it only in the leaf subclasses.