CWG 2355 Deducing noexcept-specifiers

[cpplearner]

The functionality has already been implemented (for g++ mode) in d86293c. This PR simply removes the flag to make it work in all modes.

[wchilders-nvidia]

Testing output looks like all improvements so far; we should be able to further verify and take care of updating the expected recordings if you're not able to.

[daveedvdv-nvidia]

The change looks good to me. Personal tests were okay (will need to update some).
This requires an issue number and a Changes entry.

[wchilders-nvidia]

I went ahead and gave this an issue tracking number: #197

[daveedvdv-nvidia]

Me, earlier:

The change looks good to me. Personal tests were okay (will need to update some).

I take that back: MSVC didn't implement that until very recently.
See https://godbolt.org/z/v67rKrPE9

So rather than removing the condition completely, make it dependent on ms_version_is(<1951).

Wyatt:

I went ahead and gave this an issue tracking number: #197

Thanks!

@cpplearner : Use that number for your Changes entry. It' might look something like:

10/1/26  [GH #197]
Deducing noexcept-specifiers

Previously, the front end emulated Clang and GCC's ability to deduce simple dependent
noexcept-specifiers (see the entry for EDGcpfe/18317 of 9/6/17).  Meanwhile, the standard
has been changed through Core issue 2355 to make that behavior standard-mandated.
It is now the default behavior, except in Microsoft mode with microsoft_version < 1951.

P.S.: Thanks for that contribution, @cpplearner — it may well turn out to be the first outside contribution to this project!

[cpplearner]

• Restore deduction_from_exc_spec_allowed and make it TRUE except when microsoft_version < 1951.
• Add Changes entry, wording from @daveedvdv-nvidia .

[nv-cmeerw]

Please add tests that show the change in the relevant configurations, i.e. GCC/Clang/Microsoft 19.50/Microsoft 19.51/standard mode for both C++14 and C++17. Probably just use tests/tests/cwg/2355a.sft.cpp with

//options:--c++17 -A:--c++17 --gn 160200:--c++17 --clang_version 230100:--ms_c++17 --microsoft_version 1950;fn:--ms_c++17 --microsoft_version 1951:--c++14 -A;fn:--c++14 --gn 160200;fn:--c++14 --clang_version 230100;fn:--ms_c++14 --microsoft_version 1950;fn:--ms_c++14 --microsoft_version 1951;fn
//options_all:-tused

[nv-cmeerw]

lines should wrap at column 79 (there should be a git hook pointing that out)

[cpplearner]

• Make tests/tests/cwg/2355a.sft.cpp test relevant configurations.
• Wrap lines in Changes to fit in 80 columns.
• Remove the CWG 2355 tests from tests/expectations. These tests are no longer expected to FAIL.

[daveedvdv-nvidia]

Looks good to me — thanks!

[daveedvdv-nvidia]

Merge with f0e30b6