Re: [PATCH RFC] c++: streamline process for adding new builtin trait

[Marek Polacek <polacek@redhat.com>]

On Thu, Sep 29, 2022 at 11:05:04AM -0400, Patrick Palka via Gcc-patches wrote:
> Adding a new builtin trait currently involves some boilerplate (as can
> be seen in r13-2956-g9ca147154074a0) of defining corresponding RID_ and
> CPTK_ enumerators and adding them to various switch statements across
> many files.  The exact switch statements we need to change is determined
> by whether the proposed trait yields a type or an expression.
> 
> This RFC patch attempts to streamline this process via a centralized
> cp-trait.def file for declaring the important parts about a builtin trait
> (whether it yields a type or an expression, its code, its spelling and
> its arity) and using this file to automate away the switch statement
> addition boilerplate.  It also converts 9 traits to use this approach
> by way of example (we can convert all the traits once the design is
> settled).
> 
> After this change, the process of adding a new builtin trait is just
> (modulo tests): declare it in cp-trait.def, define its behavior in
> finish_trait_type/expr, and handle it in diagnose_trait_expr if it's
> an expression-yielding trait (this last step is unfortunate but since
> the switch has no default case, we'll at least get a diagnostic if we
> forget to do it).
> 
> Does this look like a good approach?

I think it'd be fantastic to have this.  It's been very easy to forget
to update pp_cxx_trait, or names_builtin_p.  cp-trait.def just needs to
describe what the arguments mean.
 
Marek