Seems like an off-by-one error in checking the bounds.

import std.stdio, std.conv;

void main() {
    auto foo = cast(ubyte) roundTo!uint(255.0);
    writeln(foo);  // 255:  Works.

    auto bar = roundTo!ubyte(254.0);
    writeln(bar);  // 254:  Works.

    auto baz = roundTo!ubyte(255.0);  // Throws
}