This patch make G++03 explicitly unsupported with libc++, as discussed on the mailing lists.
Below is the rational for this decision.
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 . 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  . Compared to libstdc++, debug information size increases of +10% are not uncommon.