This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
[Moved to DR at the October, 2012 meeting.]
A constexpr constructor is required to initialize all non-static data members (9.2.6 [dcl.constexpr] paragraph 4), which conflicts with the requirement that a constructor for a union is permitted to initialize only a single non-static data member (11.9.3 [class.base.init] paragraph 8).
Proposed resolution (February, 2012):
Change 9.2.6 [dcl.constexpr] paragraph 4 as follows:
In a definition of a constexpr constructor, each of the parameter types shall be a literal type. In addition, either its function-body shall be = delete or = default or it shall satisfy the following constraints:
every non-static data member and base class sub-object shall be initialized (11.9.3 [class.base.init]);