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
Comment #1 by k.hanazuki — 2012-03-13T21:21:44Z
Forgot to mention there is the same problem with opEquals and ==/!=.
Comment #2 by k.hanazuki — 2012-03-13T23:29:50Z
ah, my idea improves nothing than allowing rvalue-to-const-ref conversion. forget about it.
Comment #3 by robert.schadek — 2024-12-13T17:59:12Z