[Bug c++/78948] [C++17] constexpr if instantiating too eagerly

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78948

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
During parsing, we call mark_used on the function to call, which in turn does
instantiate_decl -> add_pending_template.
So, shall we avoid calling mark_used from parser->in_discarded_stmt (we'd need
to propagate that flag down to call.c), something different?