Why are unsigned to signed conversions implicit and don't emit a warning?

[Andrej Mitrovic]

I just had a little bug in my code. In the WindowsAPI, there's this alias:

alias ubyte BYTE;

Unfortunately I didn't check for this, and I erroneously assumed BYTE was a 
signed value (blame it on my lack of coffee). So when I used code like this:

alias Tuple!(byte, "red", byte, "green", byte, "blue") RGBTuple;

RGBTuple GetRGB(COLORREF cref)
{
    RGBTuple rgb;
    rgb.red   = GetRValue(cref);
    rgb.green = GetGValue(cref);
    rgb.blue  = GetBValue(cref);
    
    return rgb;
}

The rgb fields would often end up being -1 (Yes, I know all about how signed vs 
unsigned representation works). My fault, yes.

But what really surprises me is that these unsigned to signed conversions 
happen implicitly. I didn't even get a warning, even though I have all warning 
switches turned on. 

I'm pretty sure GCC would complain about this in C code. Visual Studio 
certainly complains if I set the appropriate warnings, examples given:

warning C4365: '=' : conversion from 'unsigned int' to 'int', signed/unsigned 
mismatch
warning C4365: '=' : conversion from 'unsigned short' to 'short', 
signed/unsigned mismatch