为什么不能在Dart中分配泛型变量？

void test<T1 extends num, T2 extends String>(T2 arg) {
    //    ↓ A value of type 'int' can't be assigned to a variable of type 'T1'.  Try changing the type of the variable, or casting the right-hand type to 'T1'.
    T1 t = 0;

    //     ↓↓↓↓ A value of type 'String' can't be assigned to a variable of type 'T2'.  Try changing the type of the variable, or casting the right-hand type to 'T2'.
    arg = 'Test';
}