Update libstdc++ hack comment

[urnathan]

Summary

This libstc++ hack isn't ready for removal. Updating the comment to note what I found. While I have not proven Ville's __is_throw_swappable patch made this go away, that patch did remove the use of noexcept(noexcept(swap(....))). I'm not sure when gcc grew deferred noexcept parsing.

[bruno]

LGTM

[mizvekov]

Here are my findings around this issue: https://godbolt.org/z/TE9rdxae8

So GCC started parsing this correctly from 9.4 to 10.1.
It does not look like they have a workaround for their own older libstdc++.

It seems to me this HACK is ready for removal. Is there something else I might be missing?

[urnathan]

In D101441#2877463, @mizvekov wrote:

It seems to me this HACK is ready for removal. Is there something else I might be missing?

we want to support all the way back to gcc 4.8.3, IIRC that's the version in CentOS $SOMEPOPULARVERSION

see https://reviews.llvm.org/D100465 for that documentation

clang/lib/Sema/SemaExceptionSpec.cpp

if (const PointerType *PtrTy = T->getAs<PointerType>())
     T = PtrTy->getPointeeType();
   else if (const ReferenceType *RefTy = T->getAs<ReferenceType>())
     T = RefTy->getPointeeType();
   else if (const MemberPointerType *MPTy = T->getAs<MemberPointerType>())
     T = MPTy->getPointeeType();
   return T->getAs<FunctionProtoType>();
 }
 
-/// HACK: libstdc++ has a bug where it shadows std::swap with a member
-/// swap function then tries to call std::swap unqualified from the exception
-/// specification of that function. This function detects whether we're in
-/// such a case and turns off delay-parsing of exception specifications.
+/// HACK: 2014-11-14 libstdc++ had a bug where it shadows std::swap with a
+/// member swap function then tries to call std::swap unqualified from the
+/// exception specification of that function. This function detects whether
+/// we're in such a case and turns off delay-parsing of exception
+/// specifications. Libstdc++ 6.1 (released 2016-04-27) appears to have
+/// resolved it as side-effect of commit ddb63209a8d (2015-06-05).
 bool Sema::isLibstdcxxEagerExceptionSpecHack(const Declarator &D) {
   auto *RD = dyn_cast<CXXRecordDecl>(CurContext);
 
   // All the problem cases are member functions named "swap" within class
   // templates declared directly within namespace std or std::__debug or
   // std::__profile.
   if (!RD || !RD->getIdentifier() || !RD->getDescribedClassTemplate() ||
       !D.getIdentifier() || !D.getIdentifier()->isStr("swap"))