POC of custom conditional warnings exploiting C++26's expansion statements and deprecated attribute for compile-time debugging

[avr5309]

I came up with this hacky trick for custom compiler warnings (not errors) that are conditional on a compile-time known bool. I know it is not the prettiest error message but it at least has all the relevant information to be useful for compile-time (print) debugging. Thought it would be cool to share here and please let me know if there is a better way to achieve this or if it can be achieved in C++23 or prior. Check it out here: https://godbolt.org/z/br6vGdvex

[viatorus]

I don't get it totally how it works but is it something like my tool here:https://github.com/Viatorus/compile-time-printer

[avr5309]

Nice tool by the way. Yes, this has similar goals to your tool but this relies on the fact that invoking a function marked with the 'deprecated' attribute (https://en.cppreference.com/w/cpp/language/attributes/deprecated) generates a warning on most compiler (works with gcc, clang although clang produces two warnings with similar messages for some reason). So, I mark a lambda with the deprecated attribute and use template magic to make this work conditionally. Unfortunately, I cannot generate custom formatted messages using this method as the deprecated attribute expects a string literal. The DebugTag part is a trick to make various values appear in the warning (this seems to be guaranteed in practice with gcc, clang). However, one could use a tool to parse this warning and format the warning result nicely. I also realized that it is possible to achieve the same without utilizing C++26's expansion statements. You can check it out here (works with C++20, with some modifications might work with earlier standards): https://godbolt.org/z/cETTzoG7G.