Jan Engelhardt [off-list ref] wrote:

"""it [the array size expression] shall be a converted constant expression of
type std::size_t and its value shall be greater than zero."""
—http://eel.is/c++draft/dcl.array

Interesting.  You're not actually quoting the full sentence:

	If the constant-expression is present, it shall be a converted
	constant expression of type std​::​size_­t and its value shall be
	greater than zero.

This suggests that:

	__u64 ptr[]

is actually valid since:

	D1 [ constant-expressionopt ] attribute-specifier-seqopt

suggests that the part between the brackets is optional.

David