[PATCH] [libc++] In C++11 GNU extension mode, start emulating libstdc++'s use of constexpr.

[Richard Smith]

On Wed, Nov 27, 2013 at 3:31 PM, Howard Hinnant <howard.hinnant at gmail.com>wrote:

> On Nov 26, 2013, at 5:04 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>
> > Hi rsmith, howard.hinnant,
> >
> > libstdc++ always declares various entities as constexpr if C++14 makes
> > them constexpr, even in C++11 mode. To support programs which rely on
> this
> > behavior, start applying constexpr to those entities in C++11 mode only
> if
> > GNU extensions are enabled (i.e. -std=gnu++11).
> >
> > We have to be selective about which entities we do this for because of
> the
> > relaxed constexpr rules in C++14.
> >
> > This patch applies the change to entities in <utility>, as well as its
> > dependencies, std::move and std::forward.
> >
> > http://llvm-reviews.chandlerc.com/D2276
> >
> > Files:
> >  include/__config
> >  include/__tuple
> >  include/type_traits
> >  include/utility
> >  test/utilities/utility/pairs/pair.astuple/get_const.pass.cpp
> >  test/utilities/utility/pairs/pair.astuple/get_non_const.pass.cpp
> >  test/utilities/utility/pairs/pair.astuple/pairs.by.type1.fail.cpp
> >  test/utilities/utility/pairs/pair.astuple/pairs.by.type2.fail.cpp
> >  test/utilities/utility/pairs/pair.astuple/pairs.by.type3.fail.cpp
> >
>  test/utilities/utility/pairs/pairs.pair/const_first_const_second.pass.cpp
> >  test/utilities/utility/pairs/pairs.pair/const_pair_U_V.pass.cpp
> >  test/utilities/utility/pairs/pairs.pair/copy_ctor.pass.cpp
> >  test/utilities/utility/pairs/pairs.pair/default.pass.cpp
> >  test/utilities/utility/pairs/pairs.spec/comparison.pass.cpp
> >  test/utilities/utility/pairs/pairs.spec/make_pair.pass.cpp
> > <D2276.1.patch>
>
> I find myself wondering if the following would not be simpler and more
> flexible:
>
> In <__config>:
>
> #if _LIBCPP_STD_VER <= 11
> #  ifndef _LIBCPP_CONSTEXPR_AFTER_CXX11
> #    define _LIBCPP_CONSTEXPR_AFTER_CXX11
> #  endif
> #  define _LIBCPP_EXPLICIT_AFTER_CXX11
> #  define _LIBCPP_DEPRECATED_AFTER_CXX11
> #else
> #  ifndef _LIBCPP_CONSTEXPR_AFTER_CXX11
> #    define _LIBCPP_CONSTEXPR_AFTER_CXX11 constexpr
> #  endif
> #  define _LIBCPP_EXPLICIT_AFTER_CXX11 explicit
> #  define _LIBCPP_DEPRECATED_AFTER_CXX11 [[deprecated]]
> #endif
>
> I.e. Allow the client to define _LIBCPP_CONSTEXPR_AFTER_CXX11 however he
> wants.  And if he does, his definition is respected.  Otherwise it defaults
> to how it is currently defined with _LIBCPP_STD_VER <= 11.
>

That would be problematic, because there are some places where C++1y adds
constexpr to functions where constexpr cannot be added in C++11 (non-const
member functions, functions that aren't defined as a single return
statement, and so on).
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