None of the compilers I tried accept such code:
template <int ...a> bool foo() { return (a<=> ... <=>0); }
But for any other <=,>=,==,!=,<,> it compiles.
cppreference is clear here - there is no <=> on the list of binary operators we can use for fold expression.
Is this an intentional omission in the C++ standard, or are compilers not ready with this?
The question is just pure curiosity; I just wanted to know what the C++ direction is in this area. I can imagine all other compare operators will be removed from the fold-expression list of allowed operators, as they have as much sense as <=> in a fold expression...
This is intentional.
The problem with fold-expanding comparison operators is that it works by doing this: A < B < C < D. This is only meaningfully useful in circumstances where operator< has been overloaded to mean something other than comparison. This is why an attempt was made to stop C++17 from allowing you to fold over them in the first place.
operator<=> is never supposed to be used for something other than comparison. So it is forbidden.