https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106150
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang also rejects it:
<source>:21:4: error: defaulting this default constructor would delete it after
its first declaration
S::S() = default;
^
<source>:17:8: note: default constructor of 'S' is implicitly deleted because
variant field 'v' has a non-trivial default constructor
V v;
^
