Proposed resolution (September, 2019):

Delete the indcated text from 9.2.6 [dcl.constexpr] paragraph 4:

The definition of a constexpr constructor whose function-body is not = delete shall additionally satisfy the following requirements:

• if the class is a union having variant members (11.5 [class.union]), exactly one of them shall be initialized;

• if the class is a union-like class, but is not a union, for each of its anonymous union members having variant members, exactly one of them shall be initialized;

• for a non-delegating constructor, every constructor selected to initialize non-static data members and base class subobjects shall be a constexpr constructor;

• for a delegating constructor, the target constructor shall be a constexpr constructor.