Hi,

I am doing test compiles with the experimental unreleased GCC10. And am getting some errors
that I do not see with GCC9. I am in contact with GCC maintainers, and they can not see that
this is a gcc/libstdc++ bug. However there has been some changes in libstdc++ mostly to improve/correct requirements (on types in std::variant).

I have attached sources that compiles with GCC9 but that fails with GCC10. All related to nlohmann/json and std::variant and some incomlete type being reported.

It would be nice figure out if this is a problem in nlohmann/json or in GCC10/libstdc++ before the actual release of GCC10.

This is v3.7.3 of nlohmann/json.
GCC10 as of gcc (GCC) 10.0.1 20200120 (experimental)
GCC9 as of gcc (GCC) 9.2.1 20190827 (Red Hat 9.2.1-1)

presentation.txt