[PATCH] PR libstdc++/87431 optimise valueless_by_exception()

[Jonathan Wakely]

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

        PR libstdc++/87431
        * include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
        Avoid runtime test when all alternatives are scalars and so cannot
        throw during initialization.

Tested x86_64-linux, committed to trunk.



commit 0140d5baa899fc4b6ee47636d607d8d22fc0ab5a
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Tue Sep 25 15:03:46 2018 +0100

    PR libstdc++/87431 optimise valueless_by_exception()
    
    If a std::variant can never get into valueless state then we don't need
    to do a runtime check for a valid alternative.
    
            PR libstdc++/87431
            * include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
            Avoid runtime test when all alternatives are scalars and so cannot
            throw during initialization.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 5a77e9e2d84..9289eef28cf 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -439,6 +439,8 @@ namespace __variant
       constexpr bool
       _M_valid() const noexcept
       {
+       if constexpr ((is_scalar_v<_Types> && ...))
+         return true;
        return this->_M_index != __index_type(variant_npos);
       }