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I think that would fix the issue at the type level (up to a point; for unsigned types, the "correct" type for the result of a subtraction is underdetermined -- it could be any of {same type, larger signed type, same-size signed type} depending on the circumstances).

But I think the more serious niggle is the fact that that first element shows up in the output at all. OTOH, I suppose you could write a discard_first iterator adaptor that ignores the first write and increment and passes the rest through to the underlying output iterator.

T{} is now sort of that, but it didn't exist yet when adjacent_difference was written.

Sort of. T {} is IIUC C++ "zero initialization" but this doesn't actually promise to give us the zero (additive identity) of T but instead to basically smash all T's constituent elements to zero which may not have that effect. This does what you meant for simple types like the signed integers or floats at least.

They could have added an init parameter if int() or double() isn't enough