impl(v3): remove std::future_error hack (#15904)

Author: scotthart

google/cloud/internal/future_impl.cc

@@ -17,46 +17,6 @@
 #include "google/cloud/terminate_handler.h"
 #include <stdexcept>
 
-// This function is only needed if exceptions are enabled.
-#ifdef GOOGLE_CLOUD_CPP_HAVE_EXCEPTIONS
-// TODO(#14152): Remove libC++ hack
-// The `std::future_error::future_error(std::future_errc)` constructor is not
-// guaranteed to exist until C++17. Fortunately, stdlibc++, MSVC are forgiving,
-// and libc++ is forgiving until version 18.1.
-#if GOOGLE_CLOUD_CPP_CPP_VERSION >= 201703L || _LIBCPP_VERSION < 180100
-
-namespace {
-std::future_error MakeFutureErrorImpl(std::future_errc ec) {
-  return std::future_error(ec);
-}
-}  // namespace
-
-#else
-// We can probably tolerate this terrible hack (which depends on UB) until we
-// require C++17.
-namespace {
-struct OhTheHorrors {};
-}  // namespace
-
-namespace std {
-template <>
-class promise<OhTheHorrors> {
- public:
-  static auto MakeFutureError(std::future_errc ec) {
-    return std::future_error(std::make_error_code(ec));
-  }
-};
-}  // namespace std
-
-namespace {
-std::future_error MakeFutureErrorImpl(std::future_errc ec) {
-  return std::promise<OhTheHorrors>::MakeFutureError(ec);
-}
-}  // namespace
-
-#endif
-#endif  // GOOGLE_CLOUD_CPP_HAVE_EXCEPTIONS
-
 namespace google {
 namespace cloud {
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
@@ -65,7 +25,7 @@ namespace internal {
 [[noreturn]] void ThrowFutureError(std::future_errc ec, char const* msg) {
 #ifdef GOOGLE_CLOUD_CPP_HAVE_EXCEPTIONS
   (void)msg;  // disable unused argument warning.
-  throw MakeFutureErrorImpl(ec);
+  throw std::future_error(ec);
 #else
   std::string full_msg = "future_error[";
   full_msg += std::make_error_code(ec).message();
@@ -87,7 +47,7 @@ namespace internal {
 
 std::exception_ptr MakeFutureError(std::future_errc ec) {
 #ifdef GOOGLE_CLOUD_CPP_HAVE_EXCEPTIONS
-  return std::make_exception_ptr(MakeFutureErrorImpl(ec));
+  return std::make_exception_ptr(std::future_error(ec));
 #else
   (void)ec;
   // We cannot create a valid `std::exception_ptr` in this case. It does not