[Bug c++/69384] defaulted default constructor not defined as deleted for class with a const data member which does not have a user-provided default constructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69384

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. if you just do "B b;" you get an error, but "B b{};" explicitly
zero-initializes the entire object, so nothing is left uninitialized.