[libcxx-dev] Advancing libc++ Beyond C++03

[Eric Fiselier via libcxx-dev]

Hello,

libc++ claims to support GCC with C++03 ("G++03"), and this is a problem
for our users.

Our C++03 users are all using Clang. They must be.  Less than 9% of the
C++03 tests pass with GCC [1][2]. No non-trivial C++ program could work.

Attempting to support G++03 impacts our QoI considerably. Unlike Clang,
G++03 offers almost no C++11 extensions. If we could remove all the
fallbacks for G++03, it would mean libc++ could::

* Improve Correctness:

Every `#ifdef _LIBCPP_HAS_NO_<C++11-feature>` is a bug manifest. It exists
to admit for deviant semantics.

* Achieve ABI stability between C++03 and C++11

Differences between our C++03 and C++Rest branches contain ABI bugs. For
example `std::nullptr_t` and `std::function::operator()(...)` are currently
incompatible between C++11 and C++03, but could be fixed.


* Decrease Compile Times and Memory Usage:


Writing efficient SFINAE requires C++11. Using alias templates, libc++
could reduce the number of instantiations it produces substantially.

* Decrease Binary Size


Similar to the last point, G++03 forces metaprogramming techniques that
emit more debug information [3] [4]. Compared to libstdc++, debug
information size increases of +10% are not uncommon.


I would like the communities blessing to officially unsupport GCC in C++03
in the 9.0 release.

/Eric

[1] https://gist.github.com/EricWF/83b352471c999655859f75f60c9061a8
[2] Clang and GCC are our only "supported" C++03 compilers. MSVC and XLC
don't support C++03, we don't support ILC.
[3] G++03 disallows default template parameters, so SFINAE must be written
as default function parameters. Function parameters create live variables,
live variables force debug info emission for their type, which emits debug
information for the SFINAE construct.
[4] Unlike C++03 metafunctions written with normal class templates, C++11
alias templates aren't instantiated at every use.
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