RFR: 8314488: Compile the JDK as C++17 [v6]

[Magnus Ihse Bursie]

On Thu, 11 Jan 2024 13:23:45 GMT, Julian Waters <jwaters at openjdk.org> wrote:

>> Compile the JDK as C++17, enabling the use of all C++17 language features
>
> Julian Waters has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Require clang 13 in toolchain.m4

Well, the only additional thing this PR does except raise the compiler version is to change the `--std` flag. It is a bit unclear what that means. For the JDK libraries, there are already code present that relies on C++17. For hotspot, what C++ constructions to use is strictly limited by the code standard document. As long as it does not mention any C++17 constructs, it does not really matter what the `--std` flag says. But, otoh, to be able to say something about C++17, we need first have proper support from all compilers.

So I'd say just chill a bit, give folks some time to respond. My understanding of the situation is as follows:

* Raising clang to 13.0 is uncontroversial
* Raising xlc to 17.1.1.4 seems acceptable by the folks using it (I hope I got that right)
* Raising gcc to 10.0 met some resistance. We could stop at gcc 9.0 for this PR (which is enough for C++17), and then continue discussing going to gcc 10.0 in a separate PR, or we can wait a bit more to see if @shipilev feels compelled by the arguments given in the discussion to accept going to 10.

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PR Comment: https://git.openjdk.org/jdk/pull/14988#issuecomment-1900481800