[llvm-dev] Using C++14 code in LLVM

[Zachary Turner via llvm-dev]

On Tue, Oct 31, 2017 at 3:19 PM Justin Bogner <mail at justinbogner.com> wrote:

> Zachary Turner <zturner at google.com> writes:
>
> > If 3 months later we started requiring C++17, someone could build clang 6
> > with the system compiler and then build Clang ToT.
> >
> > If 3 months later we started requiring C++20, someone could still build
> > clang 6 with the system compiler and then build Clang ToT.
> >
> > Every relaxation of the kind of code we can use in LLVM does not
> > necessitate an extra hop in the bootstrapping process, because existing
> > versions of clang can already compile through C++20.
>
> I hate to exaggerate, but this sounds almost like an argument for using
> new C++ features the day after we implement them. There obviously has to
> be some balance here.
>

Someone could probably use that line of reasoning to argue for using new
features immediately after implementing them, but that someone wouldn't be
me :)

In any case, the point was simply to illustrate that, in general, you do
not need to add a hop to the bootstrapping process every time you bump the
language standard.  C++20 is 4-6 years out before we're even discussing it
though, and any discussion we have about if, when, or how to move to it now
will probably be irrelevant by that time.

For C++14 and C++17 though, I think the argument still holds.  System
Compiler -> {GCC 7 or Clang 5-6} -> ToT

should require only 1 hop regardless of language standard, and that covers
us for quite a while.
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