Default values to bitfield elements
The syntax for bit field initialization is
struct S {int i: 5 = 42;};
and is only available in c++20: -std=c++2a
for now
No, bit-fields do not allow an initializer as part of the member declaration. You can see this in the part of the grammar that describes class members (C++11 and later, [class.mem]):
member-declarator:
declarator virt-specifier-seqopt pure-specifieropt
declarator brace-or-equal-initializeropt
identifieropt attribute-specifier-seqopt : constant-expression
The third form is the grammar for a bit-field declaration, but only the second form lists the brace-or-equal-initializer.