Inheriting constructors from Boost.Variant should be possible

[djh]

Suppose users want a strong typedef for their sumtypes. Instead of creating a weak type alias for a boost::variant the user should be able to inherit from the variant inheriting its constructors, too.

using TreeBase = boost::variant<Leaf, boost::recursive_wrapper<Node>>;

struct Tree : TreeBase {
  using TreeBase::TreeBase;

  // ...
};

Here is the full small self-contained example

​https://gist.github.com/daniel-j-h/12c8b7f1c59b5a76c7e75dab38eb06fe#file-crash-cc

GCC crashing on this is ticketed upstream

​https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78767

Creating a new type for this use-case provides the user with type safety and should be the recommended way of using variant in any non-trivial code. Unfortunately, inheriting from Boost.Variant is not possible at the moment. The code example above does not compile.

In the Boost IRC channel Agustín Bergé told me this might be related to inheriting constructors with default arguments and that there are quirks related to SFINAE before C++17.

[Antony Polukhin]

Fixed by Mikhail Maximov in ​217ee7f9. Fix will be marged to master, as soon as all the tests will cycle.