d - 无法覆盖结构的 opCmp 和 opEquals

Question

来自 D 的 documentation :

Alternatively you can declare a single templated opEquals function with an auto ref parameter:

bool opEquals()(auto ref S s) { ... }

<...>

If structs declare an opCmp member function, it should follow the following form:

int opCmp(ref const S s) const { ... }

为什么下面的代码编译失败？

import std.stdio;
import std.conv;

struct Fgs {
    int v;
    this(int iv) {
        v = iv;
    }

    bool opEquals()(auto ref Fgs another) {
        return v == another.v;
    }

    int opCmp(ref const Fgs another) const {
        if (this == another) {
            return 0;
        } else if (this.v < another.v) {
            return -1;
        } else {
            return 1;
        }
    }
}


unittest {
    auto a = Fgs(42);
    auto b = Fgs(10);
    assert(a != b);
    assert(a > b);
}

这是 DMD 的输出:

/home/mfag/lighthouse/testss.d(15): Error: template testss.Fgs.opEquals does not match any function template declaration
/home/mfag/lighthouse/testss.d(10): Error: template testss.Fgs.opEquals() cannot deduce template function from argument types !()(const(Fgs))

Answer 1

const 在编写时没有考虑在内。这应该有效:

bool opEquals()(auto const ref Fgs another) const

但这是语言的一个有点不稳定的领域，将来可能会在非模板上使用 auto ref。