The [coro.generator.promise] p17 defines A as follows:

Let A be

• Allocator, if it is not void,
• Alloc for the overloads with a template parameter Alloc, or
• allocator<void> otherwise

However, it is possible that the conditions for both of the first bullets will be satisfied. In this case, the first bullet should be applied - the required expression imposes convertibility.

Proposed resolution:
Modify 17.1 by adding "otherwise" after coma in 17.