Why does compilation fail only if I use lexy::parse and not lexy::parse_as_tree?

[EmmanuelMess]

Here is a working example of the error. If I uncomment dummy() the program no longer compiles, but I am not sure what broke.

[foonathan]

You've only used lexy::parse_as_tree before, which completely ignores all value callbacks. With lexy::parse reading from a file, the program no longer compiles because you are now instantiating the callbacks.

Your issue is that the Static production produces a value of the Static tag type, but nobody uses it. You can either use lexy::noop as the value to not produce a tag type and drop it entirely (https://godbolt.org/z/nPh1dM9aq), store the tag type in the datastructures, or just inline the rule from Static anywhere its used.

[EmmanuelMess]

The error needs to be more clear IMO 😅

[foonathan]

If we skip the template callstack, the error message is:

/opt/compiler-explorer/libs/lexy/trunk/include/lexy/grammar.hpp:355:36: error: static assertion failed: missing value callback overload for production
  355 |             static_assert(_detail::error<Production, Args...>,
      |                           ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/lexy/trunk/include/lexy/grammar.hpp:355:36: note: 'lexy::_detail::error<internal::grammar::StaticQualifiedPoint, internal::ast::Static, lexy::member<lexy::_mem_ptr_fn<&internal::ast::StaticQualifiedPoint::identifier> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >' evaluates to false

This tells you that the callback of StaticQualifiedPoint does not accept the values ast::Static, lexy::member<...>, std::string. What information would you like to have?

[EmmanuelMess]

The readable part is:

error: static assertion failed: missing value callback overload for production

Which has no info about which callback, which overload or which production (the note doesn't help with this because there is no clear indication of what check actually evaluated to false).

A better error would be:

error: static assertion failed: internal::grammar::StaticQualifiedPoint::value is missing a value callback overload for production internal::grammar::Static: valued productions that are captured in a rule cannot be ignored in value

[foonathan]

That's not possible before C++26, unfortunately. You just have to learn how to extract the relevant information from the error message and the callback and rule documentation, sorry.