Unable to use boost::multi_index::tag in module interface

[d7d1cd]

When trying to use boost::multi_index::tag in the implementation of the interface part of a C++20 module, I get a GCC error that the boost part uses internal linkage. Here's a reproducible example: https://godbolt.org/z/5e7KWj8qz

[d7d1cd]

The problem is that the boost::mpl::aux::ptr_to_ref function template is declared as static. Is there a reason for this, or is it just a bug?

[vinniefalco]

Someone is still using MPL ?

[d7d1cd]

Someone is still using MPL ?

Boost itself uses MPL in its implementation of multi_index::tag

[d7d1cd]

To solve the problem, I made a PR boostorg/mpl#90