commit 385dcdd9be5b3c838a23c2035e0eaad72c0229a0
author Václav Kubernát <kubernat@cesnet.cz> Wed Nov 25 17:46:54 2020 +0100
committer Václav Kubernát <kubernat@cesnet.cz> Wed Nov 25 23:24:15 2020 +0100
tree d119ace9cb9d4bfaacc80f28a44a36939dc2feeb
parent 0745618db372792262e7fc65c13f7e42071dae74

Build with C++20

Asio needs a definition to build with C++20, link to bug in comment.

The compiler also complained that it now needs full definition of enum_
inside in leaf_data_helpers.hpp:createEnum. One would think that that
would be the case even before, because the lambda uses it in a context
that requires full definition (i.e. creating an instance). However, the
lambda is a generic lambda, which means it's a template. The C++
standard says (put very simply) "compilation of templates can be
deferred until the end of a translation unit". And, because
leaf_data_helpers is a header, it is possible that the definition enum_
comes in another included file after leaf_data_helpers.hpp, which means,
by the time the compiler is compiling the generic lambda, it already has
a definition for enum_.

Now, in C++20, std::transform is constexpr. The standard says, that
compilers can NOT use this deferring mechanic when dealing with
constexpr function, so it needs the definition immediately, which means
that ast_values.hpp has to be included inside leaf_data_helpers.

This probably wouldn't have happened if I defined the function inside a
cpp file (even clang-tidy warns for this).

tl;dr: the generic lambda in createEnum didn't need the enum_ definition
immediately, but now, std::transform is constexpr and that means the
definition IS needed immediately

https://timsong-cpp.github.io/cppwp/n4659/temp.point#8 is where it says
about the templates. Thanks to PJBoy on freenode.#c++ for explaining
this to me.

Change-Id: I4675643572db37233a88f7a7ee3570fabb897dad