RFC: Bump toolchain requirements

Clang Frontend
1

Hi,

the policy for updating the minimum compiler versions able to build llvm (and as a side-effect, the option of using features of a newer C++ standard) is here:

  LLVM Developer Policy — LLVM 16.0.0git documentation

The current compiler requirements are documented here:

  Getting Started with the LLVM System — LLVM 16.0.0git documentation

I don't know if buildbots verify those versions.

I'm wondering if anyone has strong feelings for or against bumping the minimum compiler requirements to:

* GCC 7.4 (Dec 2018 - GCC 7 Release Series - GNU Project)
* Clang 6.0 (Mar 2018 - [llvm-announce] LLVM 6.0.0 Release)
* MSVC 2019
* AppleClang ??

Those versions of GCC and Clang are available on Ubuntu 18.04 LTS.

  Ubuntu – Details of package clang in bionic-updates
  Ubuntu – Details of package gcc in bionic-updates

Ubuntu 16.04 LTS is end-of-standard-support this month:

  Releases - Ubuntu Wiki

Those compilers have much-improved C++17 support compared to the current baseline.

Thanks,

Stephen.

2

The policy for Visual Studio has been to support the two last major
releases and I don't see a reason to deviate from it. The next major
release will be Visual Studio 2022 [1].

Michael

[1] Visual Studio 2022 - Visual Studio Blog

3

Can't say I have any strong feelings on this - if it's consistent with
the policy (is it?) & there's something to be gained by it (sounds
like it), great.

(might want to check this recent thread
[cfe-dev] Oldest supported libstdc++ in
case there's anything interesting there - and maybe summarize some/any
of the previous discussions to bump the minimum version and why any
counterarguments in those discussions are now addressed)