[Bug c++/81852] New: Feature request: __cpp_threadsafe_static_init

["daniel.kruegler at googlemail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81852

            Bug ID: 81852
           Summary: Feature request: __cpp_threadsafe_static_init
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: daniel.kruegler at googlemail dot com
  Target Milestone: ---

Since the recent update of "Feature-testing recommendations for C++",

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0096r4.html#recs.cpp11

the feature macro "__cpp_threadsafe_static_init" has been recommended for C++
language level c++11 on. This is extremely useful to have for code that has to
program defensively against the possibility that thread-safe static locale
initialization has been deactivated (via -fno-threadsafe-statics). With that
feature macro available, portable code can be written that always provides a
thread-safe static initialization, for example by means of std::call_once as
the following excerpt demonstrates:

Thingy& get_thing()
{
#ifndef __cpp_threadsafe_static_init
    [...]
    static std::once_flag flag;
    std::call_once(flag, init::get);
    return init::get();
#else
    static Thingy result;
    return result;
#endif
}

Current clang HEAD 6.0.0 (https://github.com/llvm-mirror/clang.git 43) has
already provided support for this.

A test-case would be that given the compiler flags

$ g++ prog.cc -Wall -Wextra -std=c++11 -pedantic 

the following would be well-formed:

//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
#ifndef __cpp_threadsafe_static_init
# error "__cpp_threadsafe_static_init not defined"
#endif

int main() 
{
}
//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

while given the compiler flags

$ g++ prog.cc -Wall -Wextra -std=c++11 -pedantic "-fno-threadsafe-statics"

the same code should be ill-formed:

prog.cc:2:3: error: #error "__cpp_threadsafe_static_init not defined"
 # error "__cpp_threadsafe_static_init not defined"
   ^~~~~