Inconsistent abilities to compare const/immutable classes

[dlangBugzillaToGithub]

Denis Shelomovskii (@denis-sh) reported this on 2013-11-23T23:50:11Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=11588

Description

Currently because of Issue 1824 `opEquals` and `opCmp` aren't `const`.

dmd rejects `opCmp` calls but accepts `opEquals` ones:
---
class C
{
    int i;
    override bool opEquals(Object o) { ++i; return true;}
    override int opCmp(Object c) { ++i; return 0; }
}

void main()
{
    const c1 = new C, c2 = new C;
    assert(c1 == c2); // ok, calls `opEquals`
    assert(c1 <= c2); // Error: mutable method main.C.opCmp is not callable using a const object
}
---

Both should be either accepted or rejected. See Issue 11557 for proposal to allow `opCmp` calls too.