<div dir="ltr"><div><div></div></div><div>Exposing some extensions beyond the standard is the intent behind there being a separate -std=gnu++17 flag, vs -std=c++17, so that doesn't really feel like a cryptic behavioral change to me for libc++ to expose the no-longer-standard functions only in -std=gnu++17.</div><div><br></div><div>But it'd also be fine by me to just keep them enabled by default in all modes -- the important part is that they get enabled in the default compilation mode. Regarding opt-out -- we could perhaps honor the flag being set explicitly to zero, e.g. -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES=0.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Jun 4, 2020 at 11:24 AM Shoaib Meenai <<a href="mailto:smeenai@fb.com" target="_blank">smeenai@fb.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal">Although I understand and support the rationale here, I’d still like some way to opt out of libc++ exposing these removed features. For us, the C++17 upgrade was an excellent opportunity to find and modernize all these issues in our codebase.
(I know that ideally you should be doing that on a more regular basis, and that e.g. there’s clang-tidy tools to help with that, but the upgrade was a nice forcing function.)<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in"><b><span style="font-size:12pt;color:black">From:
</span></b><span style="font-size:12pt;color:black">libcxx-dev <<a href="mailto:libcxx-dev-bounces@lists.llvm.org" target="_blank">libcxx-dev-bounces@lists.llvm.org</a>> on behalf of Eric Fiselier via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a>><br>
<b>Reply-To: </b>Eric Fiselier <<a href="mailto:eric@efcs.ca" target="_blank">eric@efcs.ca</a>><br>
<b>Date: </b>Thursday, June 4, 2020 at 1:09 AM<br>
<b>To: </b>James Y Knight <<a href="mailto:jyknight@google.com" target="_blank">jyknight@google.com</a>><br>
<b>Cc: </b>Libc++ Dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [libcxx-dev] Enable _LIBCPP_ENABLE_*_REMOVED_* by default in -std=gnu++NN?<u></u><u></u></span></p>
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<p class="MsoNormal" style="margin-left:0.5in">On Wed, Jun 3, 2020 at 8:11 PM James Y Knight via libcxx-dev <<a href="mailto:libcxx-dev@lists.llvm.org" target="_blank">libcxx-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">It seems to me like it may be reasonable to increment the default language version in Clang from -std=gnu++14 to -std=gnu++17 soon.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">However, one issue is that a <i>lot</i> of code is broken by libc++'s removal of auto_ptr, random_shuffle, bind1st/bind2nd<span style="color:black">, ptr_fun/mem_fun/</span>mem_fun_ref/etc. (And almost nothing is
broken by the removal of {set_,get_,}unexpected, but I'm not sure it makes sense to actually treat that differently.)<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">Given:<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">1. There is a large amount of existing usage of these functions/classes.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">2. GCC has not, and has no plans to, remove them from libstdc++.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">3. We ought to continue incrementing the default C++ language version in clang.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">4. It would be good not to break vast swaths of existing code while doing so. <u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">I propose that we modify the libc++ configuration to define _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES unless __STRICT_ANSI__ is defined.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">I agree that libc++ should act pragmatically and allow users to upgrade without the additional cost of fixing usages of removed features.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">I have some concerns with using `__STRICT_ANSI__`<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">Currently, there is no distinction in libc++ between `-std=c++XX` and `-std=gnu++xx`; I don't want to double the number of supported<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">configurations, nor do I want something as cryptic as the difference between `gnu++XX` and `c++XX` to cause behavioral changes in libc++.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">In the past libc++ has been bitten by similar behavior changes when `-pedantic` is specified. This feels like it will cause the same sorts of problems. <u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">That define is set only if the user explicitly passes --std=c++XX. In the default compilation mode, the removed functionality would continue to be available, and only users who ask for a strict compliance mode would
get an error.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">I think that would strike a reasonable balance between not unnecessarily breaking code, and moving things forward.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">For libc++ and it's maintainers, there is no balance to be struck here, and there never will be. I believe the library has already committed to supporting the removed features ad infinitum.<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">So what cost do we bear for exposing them differently?<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">Are there other suggestions for how to best gate these features?<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">Thoughts?<u></u><u></u></p>
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<p class="MsoNormal" style="margin-left:0.5in">_______________________________________________<br>
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