import std.traits;

void foo(int) { }
void bar(ref int) { }

void main()
{
   pragma(msg, ParameterTypeTuple!foo);  // (int)
   pragma(msg, ParameterTypeTuple!bar);  // (ref int)

   static assert(is(ParameterTypeTuple!foo ==
ParameterTypeTuple!bar));  // pass
}

If the storage classes are not compared, then ParameterTypeTuple should not keep them around. We have ParameterStorageClassTuple for that (which is itself full of bugs too).