The spec says in operatoroverloading.dd:

| Comparison operations are rewritten as follows:
|   ``a < b'' to ``a.opCmp(b) < 0'' or ``b.opCmp(a) > 0''

and

| If structs declare an opCmp member function, it should follow the following form:
|
| struct S {
|   int opCmp(ref const S s) const { ... }
| }

If you follow these specs, you cannot accept comparison between two rvalues.
For example S(0) < S(1), which is rewritten as S(0).opCmp(S(1)), is rejected _correctly_ by current DMD implementation (2.059HEAD).


In order to resolve this problem, comparison operators on struct types must be translated as follows:
  a < b
  to  (const _tmpb = b), a.opCmp(_tmpb) < 0
  or  (const _tmpa = a), b.opCmp(_tmpa) > 0