[libcxx/variant] Fix build error for when exceptions disabled.

[mpark]

Summary

lit -D compile_flags="-fno-exceptions" -sv test/std/utilities/variant

[zoecarver]

libcxx/include/variant

1084

Can you put a comment here?

[zoecarver]

(You still need one of the libc++ maintainers to sign off on this)

[mpark]

Added a comment.

[ldionne]

LGTM, thanks.

FYI, the right way of disabling exceptions in the test suite is

<build>/bin/llvm-lit -sv libcxx/test --param enable_exceptions=False

This will set the right Lit feature (no-exceptions).

libcxx/include/variant

if (this->valueless_by_exception() && __that.valueless_by_exception()) {
           __that);
     } else {
       __impl* __lhs = this;
       __impl* __rhs = _VSTD::addressof(__that);
       if (__lhs->__move_nothrow() && !__rhs->__move_nothrow()) {
         _VSTD::swap(__lhs, __rhs);
       }
       __impl __tmp(_VSTD::move(*__rhs));
-      static constexpr bool __is_noexcept =
-#ifdef _LIBCPP_NO_EXCEPTIONS
-          true;
-#else
-          __all<(is_nothrow_move_constructible_v<_Types> &&
-                 is_nothrow_swappable_v<_Types>)...>::value;
-#endif
-      if constexpr (__is_noexcept) {
+#ifndef _LIBCPP_NO_EXCEPTIONS
+      if constexpr (__all<(is_nothrow_move_constructible_v<_Types> &&
+                           is_nothrow_swappable_v<_Types>)...>::value) {
         this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
       } else {
         // EXTENSION: When the move construction of `__lhs` into `__rhs` throws
         // and `__tmp` is nothrow move constructible then we move `__tmp` back
         // into `__rhs` and provide the strong exception safety guarantee.
         try {
           this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
         } catch (...) {
           if (__tmp.__move_nothrow()) {
             this->__generic_construct(*__rhs, _VSTD::move(__tmp));
           }
           throw;
         }
       }
+#else
+      // this isn't consolidated with the `if constexpr` branch above due to
+      // `throw` being ill-formed with exceptions disabled even when discarded.
+      this->__generic_construct(*__rhs, _VSTD::move(*__lhs));
+#endif
       this->__generic_construct(*__lhs, _VSTD::move(__tmp));
     }
   }
 
 private:
   inline _LIBCPP_INLINE_VISIBILITY
   bool __move_nothrow() const {
     constexpr bool __results[] = {is_nothrow_move_constructible_v<_Types>...};