Undefined reference to static constexpr with clang with no optimisation (Job Failed #58344)

Job #58344 failed for afaa9cd7:

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.

I'm testing this solution, I will add it to !6 (merged) if the CI passes and I can find enough information to be confident it's indeed the problem and a correct solution.