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.

[Adopted as a DR at the November, 2019 meeting.]

Paper P1331R2 removed the requirement that a constexpr constructor initialize every non-variant non-static data member, but it left untouched the corresponding requirements for variant members. That is, the modified text in 9.2.6 [dcl.constexpr] paragraph 4 still contains:

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;

Presumably this was an oversight and these two bullets should be changed from “exactly” to “at most” or something similar.