[Issue 10567] New: Typeinfo.compare has unreasonable signature requirements on opCmp

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10567

           Summary: Typeinfo.compare has unreasonable signature
                    requirements on opCmp
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: hsteoh at quickfur.ath.cx


--- Comment #0 from hsteoh at quickfur.ath.cx 2013-07-07 21:25:25 PDT ---
CODE:

------------snip-----------
import std.stdio;

struct S {
        int[] data;

        int opCmp(const S s) const {
                return (data < s.data) ? -1 : (data == s.data) ? 0 : 1;
        }
}

void main() {
        auto s = S([1,2,3]);
        auto t = S([1,2,3]);

        writeln(s==t);
        writeln(typeid(s).compare(&s, &t)); // prints 16
}
------------snip-----------

Here, we defined opCmp to compare the array wrapped in S, and == correctly
calls the custom opCmp to return true.

However, typeid(S) fails to call the custom opCmp; it appears to fall back to
the default implementation of opCmp, which does a bitwise compare of S. This is
a bug, because if the signature of opCmp is changed to:

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

then typeid(S) correctly calls the custom opCmp instead.

However, requiring ref in the argument is unnecessarily restrictive. If ==
works correctly without requiring a ref const argument, then why should
typeid(S).compare require a ref const argument?

This bug is blocking issue #8435 and issue #10118.

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