`PolymorphicValue` incurs some quite confusing requirements due to
the clumsy way how the instance handling and copy support must be injected.
As pointed out in #1197, with modern C++, this could be much simplified.
An overhaul of `PolymorphicValue` seems urgent thus -- in the meanwhile
we need to introduce more flexibiltiy with base-class template parameters,
so that we can inject the CopySupport interface at usage site of the
complete implementation class chain. Furthermore, the size of the VTable
is added automatically to the storage requirements in `PolymorphicValue`,
so we must subtract it when specifying the payload size by a compile-time
expression based on `sizeof(ParamData<double>)`
With these tweaks, we can now keep the limit of 4 «slots» (32 bytes)
and still reserve 64bit for ID and type flags.
Ichthyostega
535e372827