Check if arg has the value arg.init.
alias T = __vector(uint[4]); assert(!T([1]).isInit); assert(T.init.isInit);
alias T = __vector(float[4]); assert(!T([1]).isInit); assert(T.init.isInit);
alias T = __vector(double[2]); assert(!T([1]).isInit); assert(T.init.isInit);
See Implementation
Check if arg has the value arg.init.