Has someone encountered similar issues with -cov?

[Guillaume Boucher via Digitalmars-d]

On Saturday, 2 July 2016 at 12:26:34 UTC, Andrei Alexandrescu 
wrote:
> How would you reshape this? It's important that the call to 
> hook is physically at the end of the function and issued just 
> in that place, and that the code does not do any redundant work.

Your function does redundant work.  E.g. opCast!(int, ubyte) 
should not require any checks.  I also don't understand why 
opCast!(int, ubyte) is not allowed.

The following code should do the same as yours, but without 
unnecessary checks:

U opCast(U, T)(T payload)
{
         import std.traits;
         enum Tsizeof = is(T==bool) ? 0 : T.sizeof;
         enum Usizeof = is(U==bool) ? 0 : U.sizeof;
         enum noCheck = isUnsigned!T == isUnsigned!U && Tsizeof <= 
Usizeof ||
                        isUnsigned!T && Tsizeof < Usizeof;
         enum checkPayload = !isUnsigned!T && isUnsigned!U;
         enum checkResult = isUnsigned!T && !isUnsigned!U;
         static if (checkResult)
         {
                 static assert(U.sizeof <= T.sizeof);  // I don't 
understand this
         }

         if (!checkPayload || payload >= 0)
         {
                 auto result = cast(U) payload;
                 if (noCheck || result == payload && (!checkResult 
|| result >= 0))
                         return result;
         }
         return hook!U(payload);
}