[Issue 8681] dmd accepts mutable AA key types for objects

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8681


hsteoh at quickfur.ath.cx changed:

           What    |Removed                     |Added
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                 CC|                            |hsteoh at quickfur.ath.cx


--- Comment #1 from hsteoh at quickfur.ath.cx 2013-07-08 20:10:39 PDT ---
There's a case where accepting mutable keys may be OK:

class C {
    immutable int x;
    int y, z;

    this() { ... /* initialize x */ }

    size_t toHash() const {
        // The "real" data is in x; y and z are used for, e.g., caching
        // or holding temporary values
        return typeid(int).getHash(x);
    }
    bool opEquals(const C c) const {
        return x == c.x;
    }
}

Since both toHash and opEquals only depend on C.x, it should be permissible to
accept mutable C as AA key; the immutability of C.x guarantees we won't run
into problems.

The problem is, I don't think such a case is expressible in the current type
system.

(Though arguably, such cases are probably bad design and shouldn't be
encouraged/catered to.)

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