[patch] libstdc++/64650 add bad_optional_access default constructor

[Jonathan Wakely]

On 19/01/15 15:35 +0000, Jonathan Wakely wrote:
>The Library Fundamentals TS says
>std::experimental::bad_optional_access should have a default
>constructor, but we only support construction from strings.
>
>This removes the unused and non-standard std::string constructor and
>adds the required default constructor.
>
>Tested x86_64-linux, *not* committed.

Now committed to trunk.