Typeinfo.compare has unreasonable signature requirements on opCmp

[dlangBugzillaToGithub]

hsteoh reported this on 2013-07-07T21:25:25Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=10567

Description

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.

[dlangBugzillaToGithub]

hsteoh commented on 2013-07-07T21:31:24Z

Furthermore, if opCmp is a template function, it is never picked up in the typeinfo. This makes it impossible to make typeinfo.compare behave correctly when you need to overload opCmp on templated argument types, because an IFTI bug makes it impossible to define both a template and non-template opCmp simultaneously.

Why the big deal with typeinfo.compare? If == works, isn't that good enough? It's not good enough because the AA implementation uses typeinfo.compare for key comparisons. Thus you have the situation where two AA keys compare equal on ==, and toHash is correctly defined so that the keys have equal hash values, but aa[key] does not work because typeinfo.compare uses the wrong key comparison function. This is one of the underlying issues in issue #8435 and issue #10567.

[dlangBugzillaToGithub]

hsteoh commented on 2013-07-07T21:34:09Z

Gah, I meant issue #10118.

[dlangBugzillaToGithub]

hsteoh commented on 2013-07-08T08:33:16Z

Temporary workaround: define int opCmp(ref const T t) const *before* any of the other opCmp overloads, and have it redirect to one of them.

[dlangBugzillaToGithub]

k.hara.pg commented on 2013-07-08T19:44:29Z

Incomplete compiler fix, and supplemental druntime change.
https://github.com/D-Programming-Language/dmd/pull/2321
https://github.com/D-Programming-Language/druntime/pull/543

[dlangBugzillaToGithub]

github-bugzilla commented on 2013-07-09T02:47:53Z

Commits pushed to master at https://github.com/D-Programming-Language/druntime

https://github.com/D-Programming-Language/druntime/commit/607c25a22d8d72f2d7fb5f81c861c7e54534101e
Supplemental fix for issue 10567

Add helper function in druntime, same as opEquals case.

https://github.com/D-Programming-Language/druntime/commit/d46b4eb096c2246c59a161a1d4e5494b38d784d0
Merge pull request #543 from 9rnsr/fix10567

Supplemental fix for issue 10567

[dlangBugzillaToGithub]

github-bugzilla commented on 2013-07-21T14:02:34Z

Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/acd073afcb89d639c8c99cd7f8233788db6036d6
fix Issue 10567 - Typeinfo.compare has unreasonable signature requirements on opCmp

If needed, generate `__xopEquals` method in order to adapt `opCmp` member
function (even if it's template) to the function pointer `TypeInfo_Struct.xopCmp`.

https://github.com/D-Programming-Language/dmd/commit/19988b6d9d854f68e3984f871f0e8977a7be1f09
Merge pull request #2321 from 9rnsr/fix10567

Partial fix for Issue 10567 - Typeinfo.compare has unreasonable signature requirements on opCmp

[dlangBugzillaToGithub]

k.hara.pg commented on 2013-07-22T23:59:56Z

https://github.com/D-Programming-Language/dmd/pull/2374

[dlangBugzillaToGithub]

github-bugzilla commented on 2013-07-25T02:24:27Z

Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/f7d34abe745751326f26dc3ac203af01e151bdea
Additional fix for issue 10567 in order to generate correct TypeInfo

https://github.com/D-Programming-Language/dmd/commit/e25184bce39f87438e8170af624ea1bd35bbc7cc
Merge pull request #2374 from 9rnsr/fix10567

Additional fix for issue 10567 in order to generate correct TypeInfo

[dlangBugzillaToGithub]

hsteoh commented on 2013-07-25T11:05:34Z

Hi Kenji,

It seems that the latest pull has fixed this bug. Is there anything else that must be fixed? (You mentioned that this was only a "partial" fix?)

[dlangBugzillaToGithub]

k.hara.pg commented on 2013-08-05T02:54:04Z

(In reply to comment #9)
> Hi Kenji,
> 
> It seems that the latest pull has fixed this bug. Is there anything else that
> must be fixed? (You mentioned that this was only a "partial" fix?)

The remain issue is that does not detect alias this opCmp correctly.

struct X
{
    int opCmp(X) { return 0; }
}
struct S
{
    int val;
    X x;
    alias x this;
}
void main()
{
    S s1 = S(1);
    S s2 = S(2);
    assert(!(s1 < s2) && !(s1 > s2));   // OK
    assert(s1.opCmp(s2) == 0);          // OK
    assert(typeid(S).compare(&s1, &s2) == 0);   // doesn't work
}