Add __may_alias__ attribute to gsl::byte

[StephanDollberg]

C++20 defines std::byte* to have the same aliasing properties as
char*. Hence, we mark it with the __may_alias__ attribute under gcc
and clang.

Fixes #663

(Actually couldn't find whether this requirement is already in C++17 or not but gcc and clang already implement this in the compiler, trusting Niall on this part)

[StephanDollberg]

Ugh, apparently there is a bug in gcc < 6 with attributes on enum classes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43407

[annagrin]

@StephanDollberg Are there any tests possible to show correct code gen?

[StephanDollberg]

Had a quick look yesterday. There is already a test for aliasing but currently the tests are also being compiled with -fno-strict-aliasing. Though they don't fail if one removes that.

Will have another look today or tomorrow.

[martinmoene]

@StephanDollberg

(Actually couldn't find whether this requirement is already in C++17 or not but gcc and clang already implement this in the compiler, trusting Niall on this part)

See this remark at issue #663

Related: PR #390

[StephanDollberg]

Ah great, somehow missed that comment. Will amend the commit message.

…

On Thu, 26 Apr 2018, 08:37 Martin Moene, ***@***.***> wrote: @StephanDollberg <https://github.com/StephanDollberg> (Actually couldn't find whether this requirement is already in C++17 or not but gcc and clang already implement this in the compiler, trusting Niall on this part) See this remark at issue #663 <#663 (comment)> Related: PR #390 <#390> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#668 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACxSk72nlV_V73aSc6YYB_frhfLa6kySks5tsXlGgaJpZM4Teag0> .

[StephanDollberg]

@annagrin Checked with the tests. So the other day I was compiling with clang which apparently doesn't warn. GCC requires -fstrict-aliasing or -O2 (which turns strict aliasing on) to warn about the aliasing test.

Given that gcc 5 doesn't support the attribute I'd recommend against turning those flags on for now.

Rebased as well.

[annagrin]

Thanks @StephanDollberg!