[v3 PATCH] LWG 2510, make the default constructors of library tag types explicit.

[Dominique d'Humières]

Revision r230175

> 2015-11-10  Ville Voutilainen  <ville.voutilainen@gmail.com>
>
>     LWG 2510, make the default constructors of library tag types
>     explicit.
>     * include/bits/mutex.h (defer_lock_t, try_lock_t,
>     adopt_lock_t): Add an explicit default constructor.
>     * include/bits/stl_pair.h (piecewise_construct_t): Likewise.
>     * include/bits/uses_allocator.h (allocator_arg_t): Likewise.
>     * libsupc++/new (nothrow_t): Likewise.
>     * testsuite/17_intro/tag_type_explicit_ctor.cc: New.

 breaks bootstrap

libtool: compile:  /opt/gcc/build_w/./gcc/xgcc -shared-libgcc -B/opt/gcc/build_w/./gcc -nostdinc++ -L/opt/gcc/build_w/x86_64-apple-darwin14.5.0/libstdc++-v3/src -L/opt/gcc/build_w/x86_64-apple-darwin14.5.0/libstdc++-v3/src/.libs -L/opt/gcc/build_w/x86_64-apple-darwin14.5.0/libstdc++-v3/libsupc++/.libs -B/opt/gcc/gcc6w/x86_64-apple-darwin14.5.0/bin/ -B/opt/gcc/gcc6w/x86_64-apple-darwin14.5.0/lib/ -isystem /opt/gcc/gcc6w/x86_64-apple-darwin14.5.0/include -isystem /opt/gcc/gcc6w/x86_64-apple-darwin14.5.0/sys-include -I/opt/gcc/work/libstdc++-v3/../libgcc -I/opt/gcc/build_w/x86_64-apple-darwin14.5.0/libstdc++-v3/include/x86_64-apple-darwin14.5.0 -I/opt/gcc/build_w/x86_64-apple-darwin14.5.0/libstdc++-v3/include -I/opt/gcc/work/libstdc++-v3/libsupc++ -D_GLIBCXX_SHARED -fno-implicit-templates -Wall -Wextra -Wwrite-strings -Wcast-qual -Wabi -fdiagnostics-show-location=once -fvisibility-inlines-hidden -ffunction-sections -fdata-sections -frandom-seed=new_handler.lo -g -O2 -std=gnu++11 -c ../../../../work/libstdc++-v3/libsupc++/new_handler.cc  -fno-common -DPIC -D_GLIBCXX_SHARED -o new_handler.o
../../../../work/libstdc++-v3/libsupc++/new_handler.cc:37:39: error: converting to 'std::nothrow_t' from initializer list would use explicit constructor 'constexpr std::nothrow_t::nothrow_t()'
 const std::nothrow_t std::nothrow = { };
                                       ^
see https://gcc.gnu.org/ml/gcc-regression/2015-11/

Dominique