This is one of those examples where I can see how it's confusing, but I just don't think you should have been doing it that way in the first place.

   bool foo = (rx & 0x0800) == 0x0800;

Not only does this not have the bug, it communicates your intent better, and modern compilers are quite capable of optimizing this.

Honestly, 8/16b ints in C and C++ are just a minefield of problems.

Here's a snippet from an open source 8051 simulator that I was debugging today:

        if( ((RAM[ACC] & 0xF0) >> 4) > 9 || 
            GetBit(RAM[PSW], CY) == 0x01 ) {
            tempAdd = RAM[ACC] + 0x60;
            RAM[ACC] = tempAdd;
            if( ((unsigned char*)&tempAdd)[1] != 0 ) {
                SetBit(RAM[PSW], CY);
            }
        }

The code is supposed to set the carry flag when RAM[ACC] + 0x60 overflows into two bytes. Can you spot the bug? It involves my favorite C++ feature: implicit conversions, and my second favorite feature: signed chars.

The bug is that when RAM[ACC] is something like 0xFF, it gets cast to (int) -1 so the the upper byte and hence the carry flag never get set.

I predict there's lots more more evil that can milked from this fount.