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

[Louis Dionne via libcxx-dev]

> On Jun 11, 2019, at 12:43, Louis Dionne via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> 
> 
>> On Jun 10, 2019, at 16:37, Eric Fiselier via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> 
>> 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::
> 
> I like the general idea and I definitely support dropping support for C++03 under GCC, since it is effectively unsupported today anyway.
> 
> 
>> * 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.
> 
> I'd like to echo what some people said regarding breaking ABI here. Instead of breaking the ABI for C++11 extensions in C++03, we could also remove them altogether. This would be a larger change that would require a deprecation period for those extensions, but it might be worth it and it would be safer than an ABI break (because source breaks are compile-time). We could:
> 
> 1. Introduce a switch that removes C++11 extensions in C++03
> 2. Somehow flag the use of these extensions, asking users to move to C++11 or use alternatives
> 3. After some time, toggle the switch to ON by default, removing the extensions
> 4. After some time, remove the switch altogether
> 5. Get rid of the C++11 extensions in C++03, which are now unused

This wasn't clear in my original message, however I would target only those C++03 extensions whose removal provides a benefit. So if a C++11 feature is implemented in C++03 as an extension but it's trivial to provide, I wouldn't necessarily try removing it per the process described above, simply because the benefit is small and we risk breaking users. I wouldn't object to the removal of such trivial extensions either, though (I like being strict about conformance).

Louis 

> 
> This hinges on the feasibility of users actually moving to C++11 or using alternatives, and most importantly the number of impacted users. If there's interest, it would be possible to survey large some code bases to get a sense for these numbers and the feasibility of my plan.
> 
> Eric, WDYT?
> 
> Either way, I would try to dissociate the two projects of dropping support for GCC/C++03 AND taking advantage of the freedom that gives us.
> 
> Louis
> 
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