According to this page, there a two definitions of a heap (used by std::make_heap, std::is_heap, etc..) :

Until C++20 :

A random access range [first, last) is a heap with respect to a
comparator comp if bool(comp(first[(i - 1) / 2], first[i])) is false
for all integer i in (​0​, last - first).

Since C++20 :

A random access range [first, last) is a heap with respect to a comparator comp if the range is a heap with respect to comp and
std::identity{} (the identity projection).

Of course, first[(i - 1) / 2] as no meaning for i = 0, so it seems that there is a mistake in the definition.

Do you have a simpler definition? For example, when comp is std::less ? I don’t understand the usage.