Yes I agree with you. Thank you for your comments! I need to be able to
define a restriction list (closed set of types) either by using/extending
existing syntax or coming up with something brand new, preferably the
former. I was looking at Constraints (
https://en.cppreference.com/w/cpp/language/constraints) but I don't have
anything concrete using that yet.

On Thu, Jun 13, 2019 at 9:34 AM Matthew Woehlke <mwoehlke.floss_at_[hidden]>
wrote:

> On 27/04/2019 09.46, Ritwik Das via Std-Discussion wrote:
> > I am not sure I understand what you mean by “class type is complete”.
>
> Practically speaking, "before the '}' of the class definition". IOW, the
> set of instantiations must be specified *inside* of the class
> definition, and becomes fixed upon the '}' of the class definition.
>
> I think what you really want isn't a virtual template, but a way to
> define an overload set which takes one or more type-variable parameters
> which "act like" template type parameters for the purpose of the
> definition, but where the overload set is otherwise just a set of
> non-template functions.
>
> IOW, you want this (strawman syntax):
>
> <template restrict typename A, restrict typename B
> : for A -> {int, float}
> : for B -> {int, float}>
> void foo(A a, B b)
> {
> ...
> }
>
> ...to be equivalent to this:
>
> void foo(int a, int b)
> {
> ...
> }
> void foo(float a, int b)
> {
> ...
> }
> void foo(int a, float b)
> {
> ...
> }
> void foo(float a, float b)
> {
> ...
> }
>
> This should also allow me to write e.g.:
>
> <template restrict typename T : for T -> {...}>
> virtual void foo(T) = 0;
>
> This lets me give a single definition, like a template would, but I'm
> not actually writing a template. It's just shorthand for a set of
> overloads with "similar" definitions (where "similar" is defined as the
> definitions being equivalent to template instantiations).
>
> I think this could actually be useful, and may be worth pursuing.
>
> --
> Matthew
>

-- 
Ritwik Das