`<complex>`: Do we still want `_Ctraits`?

[frederick-vs-ja]

It seems that <complex> can be made a bit clearer if we remove _Ctraits and some other outdated internal dispatching mechanisms. However, it is unclear whether _Ctraits is an intentional customization mechanism.

[StephanTLavavej]

I don't know what Dinkumware's original intentions were, but AFAIK we have never supported this as a user-visible customization point.

[frederick-vs-ja]

It seems that _Ctraits is useful almost only when _Ty (which is the same _Ty used in complex<_Ty>) is a non-FP type. It mainly effectively casts the inputs from _Ty to double and then casts the results from double to _Ty.

If we want to remove _Ctraits to simplify the implementation (mostly by replacing _Ctraits<_Ty>::meow with _STD meow), we'll heavily break complex<NonFP>, IIUC. I believe that we don't want to support complex<NonFP>, but is such breaking acceptable at this moment?

[StephanTLavavej]

Not for the first release of the next major version.