Variables of tuple type can be used in assignments, but not anywhere else (including initialization). And the error message is quite terrible.

void foo(T...)(T x) {
   T a;
   T b;
   a = x; // ok
   b = x; // ok
   T c = x; // fails
}

void main() {
   foo(1,2,3);
}
---
test.d(6): Error: cannot implicitly convert expression (tuple(_param_0,_param
1,_param_2)) of type (int, int, int) to int