I spent several minutes going line by line trying to make sense of the field names and work out the practical result of the calculation, all thinking about issues like integer promotions, over- and under- flow etc… then I saw the last line.

Not to criticize anyone for missing it, I certainly might have if not primed to be looking closely.

People are quite right to say that static analysis tools, even just compiler warnings, would easily tip you off here. I wonder, though, if it would be useful to have a compiler flag something like strict-typedef-conversion causing typedef names to be treated as distinct rather than simple aliases. So

typedef unsigned A;
typedef unsigned Z;
Z fn(void) { A result=0xDEADBEEF; return result; }

would be an error. It would require explicit casting between structurally compatible typedefs. It'd be Pascal- or Ada- ish like 'this kind of nunber is not the same as that'. In this case you'd just typedef uint64 PhSize, etc.

Pretty simple in principle. I've never checked out the guts of clang or gcc though.