1263666 - Add an IsConstructible trait to MFBT

Status: RESOLVED WONTFIX

(Core :: MFBT, defect)

Description

[Botond Ballo [:botond]]

C++11 has a standard type traits named |is_constructible<T, U...>| which checks whether |T| is constructible from arguments of types |U...|.

MFBT doesn't currently have an equivalent of this. We should add one. For example, we need it to implement bug 1263654.

Comment 1

[James Cheng[:JamesCheng]]

Hi Botond,
Can I take this bug and f? you?

Thanks.

Comment 2

[James Cheng[:JamesCheng]]

I briefly glance the g++ implementation, 
shall we handle all the cases that g++ implements or just handle the normal use case?

It is complicated, can we just translate the g++ implementation into gecko?

Comment 3

[Botond Ballo [:botond]]

(In reply to James Cheng[:JamesCheng] from comment #2)
> I briefly glance the g++ implementation, 
> shall we handle all the cases that g++ implements or just handle the normal
> use case?

I'm happy with just handling the most common case for now, especially as we'll hopefully be able to drop all these type trait polyfills and start using the C++11 type traits themselves soon.

> It is complicated, can we just translate the g++ implementation into gecko?

We have to be careful about copying / transliterating libstdc++ code into Mozilla code, because libstdc++ is GPL-licensed and Mozilla code is not. I think borrowing the high-level design of a component (the way I did for mozilla::Tuple) is OK, and looking at how they address a particular edge case is OK, but transliterating the entire implementation of a type trait crosses a line.

Libc++ is permissively licensed and thus doesn't have the same issue; we should be able to borrow from their implementation more liberally.

Comment 4

[Botond Ballo [:botond]]

Now that we can use C++11 standard library facilities in our code, we can just use std::is_constructible instead of writing our own.