[libc++abi] Fix LIBCXXABI_ENABLE_THREADS=OFF build

[sbc100]

Summary

This configuration was broken in rL359060.

[EricWF]

Sorry for the breakage.

Someone else noticed and proposed a fix in D61113. I think we should go with that.

The goal of this template nonsense was to avoid #ifdef-ing out as much code as possible.

[sbc100]

SGTM (I think you linked back to this issue by the way).

[EricWF]

I meant to link D61087 .

[sbc100]

Hmm.. looking at D61087 it actually looks fair bit more complex than this chance. Can we land one of them ASAP to fix the build?

libcxxabi/src/cxa_guard_impl.h

struct InitByteNoThreads : GuardObject<InitByteNoThreads> {
   void abort_init_byte() { *init_byte_address = UNSET; }
 };
 
 
 //===----------------------------------------------------------------------===//
 //                     Global Mutex Implementation
 //===----------------------------------------------------------------------===//
 
+#ifndef _LIBCXXABI_HAS_NO_THREADS
 struct LibcppMutex;
 struct LibcppCondVar;
 
-#ifndef _LIBCXXABI_HAS_NO_THREADS
 struct LibcppMutex {
   LibcppMutex() = default;
   LibcppMutex(LibcppMutex const&) = delete;
   LibcppMutex& operator=(LibcppMutex const&) = delete;
 
   bool lock() { return std::__libcpp_mutex_lock(&mutex); }
   bool unlock() { return std::__libcpp_mutex_unlock(&mutex); }
 

 template <Implementation Impl>
 struct SelectImplementation;
 
 template <>
 struct SelectImplementation<Implementation::NoThreads> {
   using type = InitByteNoThreads;
 };
 
+#ifndef _LIBCXXABI_HAS_NO_THREADS
 template <>
 struct SelectImplementation<Implementation::GlobalLock> {
   using type = InitByteGlobalMutex<
       LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
       GlobalStatic<LibcppCondVar>::instance, PlatformThreadID>;
 };
 
 template <>
 struct SelectImplementation<Implementation::Futex> {
   using type =
       InitByteFutex<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>;
 };
+#endif
 
 // TODO(EricWF): We should prefer the futex implementation when available. But
 // it should be done in a separate step from adding the implementation.
 constexpr Implementation CurrentImplementation =
 #if defined(_LIBCXXABI_HAS_NO_THREADS)
     Implementation::NoThreads;
 #elif defined(_LIBCXXABI_USE_FUTEX)
     Implementation::Futex;
 #else
     Implementation::GlobalLock;
 #endif
 
 static_assert(CurrentImplementation != Implementation::Futex
            || DoesPlatformSupportFutex(), "Futex selected but not supported");
 
 using SelectedImplementation =
     SelectImplementation<CurrentImplementation>::type;
 
 } // end namespace
 } // end namespace __cxxabiv1
 
 #endif // LIBCXXABI_SRC_INCLUDE_CXA_GUARD_IMPL_H