Proposed resolution:

Insert a new general prototype description following the current [atomics.types.operations] p. 3 as indicated:

3 [Note: Many operations are volatile-qualified. The “volatile as device register” semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. -- end note]

A::A() = default;

? Effects: Leaves the atomic object in an uninitialized state. [Note: These semantics ensure compatiblity with C. -- end note]

constexpr A::A(C desired);
[..]

Addresses US-168

[atomics.types.operations] around p. 4: The definition of the default constructor needs exposition.