Feature request: __traits(canInstantiate), like __traits(compiles) but without suppressing syntax errors

[John Colvin]

On Wednesday, 15 January 2020 at 15:13:08 UTC, FeepingCreature 
wrote:
> Right now it's impossible to check "is there a valid (template) 
> specialization for this function call" without also saying "and 
> by the way, if there's a syntax error in the function, suppress 
> the error and return false."
>
> This makes many Phobos and library functions very annoying to 
> use.
>
> A simple (?) fix for this would be adding a new trait that 
> checks if the expression resolves to a valid template 
> specialization, without attempting to also check if the 
> function body compiles, or else without suppressing internal 
> syntax errors.

Yes please (although I would say specialization and constraints, 
for all templates).

I worked around this in a commercial project a few years ago by 
creating an overload set of the passed symbol and a catch-all 
template, then instantiate and see if I got the catch-all 
(meaning no match), but obviously this is pretty nasty and 
doesn't work if one of the overloads of the original template is 
a catch-all as well.