C++11 [class.ctor]p5 says that

C++11 [class.ctor]p5 says that
  A defaulted default constructor for a class X is defined as deleted if [...]
    -  X is a union and all of its variant members are of const-qualified type.

A pedantic reading therefore says that

 union X { };

has a deleted default constructor, which is both silly and almost
certainly unintended. Pretend as if this this read

    - X is a union with one or more variant members, and all of its
      variant members are of const-qualified type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151394 91177308-0d34-0410-b5e6-96231b3b80d8