Value of type ‘T’ cannot be converted to

Even though it’s inside of an if block, the compiler doesn’t know that T is string.
Therefore, it doesn’t let you cast. (For the same reason that you cannot cast DateTime to string)

You need to cast to object, (which any T can cast to), and from there to string (since object can be cast to string).
For example:

T newT1 = (T)(object)"some text";
string newT2 = (string)(object)t;