https://d.puremagic.com/issues/show_bug.cgi?id=12024

           Summary: [REG DMD2.065-b2] template instantiation for
                    swap(SysTime, SysTime) fails
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: regression
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: sludwig@outerproduct.org


--- Comment #0 from Sönke Ludwig <sludwig@outerproduct.org> 2014-01-29 02:41:17 PST ---
Still works on 2.064. This bug in particular makes it impossible to sort an array of SysTimes or an array of structs containing SysTime. Not sure if the root cause lies in a Phobos change or in a DMD change.

---
import std.algorithm;
void main() {
  SysTime a, b;
  swap(a, b);
}
---

bug_sort.d(4): Error: template std.algorithm.swap cannot deduce function from
argument types !()(SysTime, SysTime), candidates are:
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(1997):
std.algorithm.swap(T)(ref T lhs, ref T rhs) if (allMutableFields!T &&
!is(typeof(T.init.proxySwap(T.init))))
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm.d(2042):
std.algorithm.swap(T)(T lhs, T rhs) if (is(typeof(T.init.proxySwap(T.init))))

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Vladimir Panteleev <thecybershadow@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |thecybershadow@gmail.com


--- Comment #2 from Vladimir Panteleev <thecybershadow@gmail.com> 2014-01-29 13:21:30 EET ---
Caused by https://github.com/D-Programming-Language/phobos/pull/1603

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monarchdodra@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |monarchdodra@gmail.com


--- Comment #3 from monarchdodra@gmail.com 2014-01-29 04:08:10 PST ---
> Caused by https://github.com/D-Programming-Language/phobos/pull/1603

Indeed. However, as I explained there, I think it is legit behavior:

SysTime contains a rebindable, and Rebindable contains an immutable, and swap can't make the choice to mutate immutable.

That said, the issue could be in Rebindable to being with: It stores an immutable member, but obviously mutates it all the time. In that case, why bother storing an immutable at all? I think the fix is there.

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--- Comment #4 from Vladimir Panteleev <thecybershadow@gmail.com> 2014-01-29 14:14:37 EET ---
> SysTime contains a rebindable, and Rebindable contains an immutable, and swap
can't make the choice to mutate immutable.

Actually, it contains a union between mutable and immutable. And unions imply that all guarantees are off.

So, maybe the solution would be to make swap allow an immutable field if it's in an union with a mutable field?

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--- Comment #5 from monarchdodra@gmail.com 2014-01-29 04:41:14 PST ---
(In reply to comment #4)
> Actually, it contains a union between mutable and immutable. And unions imply that all guarantees are off.
> 
> So, maybe the solution would be to make swap allow an immutable field if it's in an union with a mutable field?

There is currently (AFAIK) no way to introspect that a member is part of an anonymous union. It *could* make for a partial solution though, yes.

Also, aren't there cases where an union contains *all* const members?

In any case, yes, there is room for improvement.

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--- Comment #6 from Vladimir Panteleev <thecybershadow@gmail.com> 2014-01-29 14:59:47 EET ---
Well, the regression needs to be solved one way or another. Not being able to sort an array of timestamps is pretty bad.

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--- Comment #7 from monarchdodra@gmail.com 2014-01-29 05:18:11 PST ---
(In reply to comment #6)
> Well, the regression needs to be solved one way or another.

Absolutly.

> Not being able to sort an array of timestamps is pretty bad.

If your original issue is that of sorting timestamps, then you may also be interested in:

https://d.puremagic.com/issues/show_bug.cgi?id=11129

Because timestamps *are* assignable, so the fact that the elements aren't swappable should not be preventing you from sorting them.

I think it was when I was originally looking into that issue that I introduced that check. Small incremental improvements I guess.

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yebblies@gmail.com


--- Comment #8 from yebblies <yebblies@gmail.com> 2014-01-30 00:20:31 EST ---
(In reply to comment #5)
> 
> There is currently (AFAIK) no way to introspect that a member is part of an anonymous union. It *could* make for a partial solution though, yes.
> 

It should be possibly by comparing .offsetof with other members.

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--- Comment #9 from Kenji Hara <k.hara.pg@gmail.com> 2014-01-29 05:56:38 PST ---
(In reply to comment #3)
> > Caused by https://github.com/D-Programming-Language/phobos/pull/1603
> 
> Indeed. However, as I explained there, I think it is legit behavior:
> 
> SysTime contains a rebindable, and Rebindable contains an immutable, and swap can't make the choice to mutate immutable.
> 
> That said, the issue could be in Rebindable to being with: It stores an immutable member, but obviously mutates it all the time. In that case, why bother storing an immutable at all? I think the fix is there.

If a non-mutable field has one or more overlapped union _mutable_ fields, the whole struct is treated as modifiable.

import std.traits : Unqual;
struct Rebindable(T)
{
    union
    {
        T origin;
        Unqual!T stripped;  // overlapped union mutable field of 'origin'
    }
}
void main()
{
    Rebindable!int r1 = {origin:10};
    Rebindable!int r2 = {origin:20};
    r1 = r2;   // Rebindable!int is modifiable (assignable)
               // even if non-mutable field `T origin;` exists.
    assert(r1.origin == 20);
}

If all of overlapped union fields are non-mutable, the whole struct is not also modifiable.

struct S
{
    union
    {
        immutable int x;
        immutable int y;
    }
}
void main()
{
    S s1 = {x:10};
    S s2 = {x:20};
    s1 = s2;  // cannot modify struct s1 S with immutable members
}

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--- Comment #10 from Vladimir Panteleev <thecybershadow@gmail.com> 2014-01-29 16:05:44 EET ---
Interesting. So this should "just work":

diff --git a/std/algorithm.d b/std/algorithm.d
index 036b918..0ed5606 100644
--- a/std/algorithm.d
+++ b/std/algorithm.d
@@ -2054,6 +2054,9 @@ void swap(T)(ref T lhs, ref T rhs) if
(is(typeof(lhs.proxySwap(rhs))))
 private template allMutableFields(T)
 {
     alias OT = OriginalType!T;
+    static if (is(typeof({ T t = void; t = t; })))
+        enum allMutableFields = true;
+    else
     static if (is(OT == struct) || is(OT == union))
         enum allMutableFields = isMutable!OT && allSatisfy!(.allMutableFields,
FieldTypeTuple!OT);
     else
@@ -2072,6 +2075,9 @@ unittest
     struct S2{const int i;}
     static assert( allMutableFields!S1);
     static assert(!allMutableFields!S2);
+
+    struct S3{union X{int m;const int c;}X x;}
+    static assert( allMutableFields!S3);
 }

 unittest

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