I know how to fix interactions templates now

[FeepingCreature]

On Thursday, 1 October 2020 at 05:23:51 UTC, Stefan Koch wrote:
> The reason why templates are so bad at being composable, is 
> because template constraints don't compose.
>
> when a template T with constraints(C) yields an valid instance, 
> that proofs the constraints hold.
> which implies that for anything using an instance of T the 
> constraints C also hold.
>
> When we compose functions our constraints are coupled to types.
> Because templates themselves are (or can be) untyped in both 
> Parameters and "return value".
> We can't use those to carry the constraints forward.
>
> Leading us into a case where we have to repeat the same 
> constraints over and over again.
>
Yep.

Right now, constraints are carried forward invisibly through 
gagged template instantiation via __traits(compiles). 
Miscompilation *itself* becomes the propagated constraint 
failure, which is of course horrible.

This is why I want __traits(canInstantiate), so we can at least 
*enforce* that constraints are manually carried forward by the 
programmer. If a template passes its constraints, it should also 
pass semantic2/3 in full. It should never be necessary to gag 
errors after the constraints.

Body SFINAE is cancer.