Heisenbug where static_assert is satisfied but triggers other static_assert

Each class is handled in two passes. The second pass handles function bodies (and is called the "complete-class context"), which is why they can use class members declared below themselves.

When classes are nested, the first pass is done over the outer class and all subclasses, and then similarly the second pass.

Default member initializers are evaluated in the second pass, but your first staticassert() is handled in the first pass, so the initializers weren't seen yet at that point.

So the first assert ends up checking std::isconstructiblev somewhere deep in weaklyincrementable, sees it as false (see below), and this value is then memoized. When you then query it again after the class, you get the old value.

// One of:
iterator() noexcept = default;
iterator() noexcept(false) = default; // This works too!

The only remaning question is why do we need to know the noexceptness to check isdefault_constructible.