[Issue 20246] isCallable fails for template opCall overload

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20246

--- Comment #7 from Simen Kjaeraas <simen.kjaras at gmail.com> ---
It's impossible to solve this in the general case, and even the special cases
can't be solved in library code, but a __traits could check if the only member
of the template is a callable (potentially also if all possible members would
be callables).

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