[cfe-dev] LLVM C++14/C++17 BoF - Summary

[Zachary Turner via cfe-dev]

For those of you who were not able to attend the LLVM Developer Meeting
(and as a recap for those who were), we had a productive BoF about enabling
C++14 or C++17 in LLVM.  The outcome of this can be generally summarized as:

* There were no major objections to moving to C++14 / C++17 "as soon as
possible"
* "As soon as possible" is not immediately, but we are currently targeting
March of 2019 due to some downstream contributors' needing to resolve some
blockages before it can be possible.
* There did not seem to be a strong sentiment that we should only move to
C++14, so for the sake of this discussion we're assuming 17 unless someone
presents a strong argument why 17 is *less* desirable than 14.

Minimum Compiler Versions for "Reasonable C++17 Support" (Mostly everything
minus CTAD)
GCC - Version 7 [1]  (Version 8 for CTAD)
Clang - Version 4 [2]  (Version 5 for CTAD)
MSVC - 2017 Update 5 [3] (Update 7 for CTAD)

We plan to add a CMake warning if your compiler version is below these
versions soon.  Around January, we will promote this to a CMake error which
you must manually override by passing -DCMAKE_ALLLOW_DEPRECATED_COMPILER.

If I'm forgetting anything or misrepresenting anything that was said at the
BoF please feel free to correct me.

[1] - https://www.gnu.org/software/gcc/projects/cxx-status.html
[2] - https://clang.llvm.org/cxx_status.html
[3] -
https://docs.microsoft.com/en-us/cpp/visual-cpp-language-conformance?view=vs-2017
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