Undefined reference to static constexpr with clang with no optimisation (Job Failed #58344)
The problem is an undefined reference at link time for this constexpr.
It looks like the issue described here: as
indices is an odr-used static constexpr, in C++14 it must have a namespace scope declaration, but C++17 makes static constexpr inline by default, making this declaration useless.
What bothers me is that on my system, the undefined reference only happens with clang and with -O0, starting with -O1 vcsn compiles fine, gcc compiles fine also with any optimization level. I'm guessing that even in C++14 llvm inlines the constexpr as an optimization starting with -O1, and that gcc does it even with -O0, but I didn't confirm it.