C++ - How to check if an object has its default value at compile time?

I need to check if a generic variable (of type T) has its default value at compile time, so I wrote:

static_assert(var == T{}, "var cannot be initialized by using its default value");

However, the compiler shows me this error:

expression did not evaluate to a constant

May you help me to fix this? How should I formulate that condition so that it's accepted by static_assert?