Cannot instantiate ReturnType with lambda

[monkyyy]

On Monday, 28 July 2025 at 11:18:48 UTC, Nick Treleaven wrote:
>
> In Phobos 3 `isCallable` will likely be replaced. I think when 
> it was made to support some templates with `!()`, that muddied 
> the waters.

Whack-a-moleing; you guys do that allot

The solution I offered at the time while still fundamentally 
flawed was far more robust then what was merged