How to avoid throwing an exceptions for a built-in function?

[Stanislav Blinov via Digitalmars-d-learn]

On Wednesday, 10 May 2017 at 13:27:17 UTC, k-five wrote:

> Thanks, but I know about what are you saying. The user_apply[4] 
> has so many possibilities and I cannot use if-else

That doesn't sound right. Either you've already handled all the 
possible cases and thus expect the to! to not throw (can specify 
that i.e. via std.exception.assumeWontThrow), or, as you're 
saying, it's much more than an if-else, but in that case 
exception will be thrown on invalid input.