http://d.puremagic.com/issues/show_bug.cgi?id=6912

           Summary: non-transitivity of inout breaks const system
           Product: D
           Version: D2
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: critical
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: timon.gehr@gmx.ch


--- Comment #0 from timon.gehr@gmx.ch 2011-11-08 16:03:34 PST ---
Consider:

int[] y;

inout(const(int)[]) foo(inout(int) x){
    y=new int[10];
    const(int)[] q = y;
    inout a = q;
    return a;
}

void main(){
    immutable int x;
    immutable int[] oops = foo(x);
    assert(is(typeof(oops[0]) == immutable));
    auto oldoops_0 = oops[0];
    y[0]++;
    assert(oops[0] != oldoops_0);
}


This is caused by the fact that !is(inout(const(int)[])==inout(int[])), i.e.
inout is currently non-transitive in certain cases and making it 'override'
const transitively will fix the issue.

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Steven Schveighoffer <schveiguy@yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schveiguy@yahoo.com


--- Comment #1 from Steven Schveighoffer <schveiguy@yahoo.com> 2011-11-09 06:35:05 PST ---
inout a = q;

This line should fail to compile.  const does not implicitly cast to inout.

Note that:

immutable a = q;

doesn't work.  inout should follow the same restrictions.

I don't think this has to do with transitivity (and indeed, inout cannot override const or immutable).  It's just a simple case of inout cannot be implicitly cast from something else.

Remember, inout can get implicitly cast back to immutable or mutable upon function return.  I don't think inout should override *any* qualifiers without casts.

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timon.gehr@gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|non-transitivity of inout   |const(T[])/immutable(T[])
                   |breaks const system         |can be implicitly cast to
                   |                            |inout(const(T)[])/inout(imm
                   |                            |utable(T)[])


--- Comment #2 from timon.gehr@gmx.ch 2011-11-09 08:58:10 PST ---
(In reply to comment #1)
> inout a = q;
> 
> This line should fail to compile.  const does not implicitly cast to inout.
> 

Yes, but the issue here is that it casts to inout(const(int)[]). (dmd rejects
casting const(int[]) to inout(int[]) for exampe)

> Note that:
> 
> immutable a = q;
> 
> doesn't work.  inout should follow the same restrictions.
> 
> I don't think this has to do with transitivity (and indeed, inout cannot override const or immutable).  It's just a simple case of inout cannot be implicitly cast from something else.
> 
> Remember, inout can get implicitly cast back to immutable or mutable upon function return.  I don't think inout should override *any* qualifiers without casts.

You are right, it is not a transitivity/overriding issue.

Currently inout(const(T)) == inout(immutable(T)) == inout(T). That is bad, for
example, how to represent the element type of inout(const(char)[]) ? (DMD
treats it as const(char), which is obviously wrong, it should be
inout(const(char)).)

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Kenji Hara <k.hara.pg@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch, rejects-valid
           Platform|Other                       |All


--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-09 21:38:49 PST ---
https://github.com/D-Programming-Language/dmd/pull/504

This patch will forbid some conversions like follows.
e.g. const(int)[]    to inout(const(int)[])
     const(int)[int] to inout(const(int)[int]).

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Steven Schveighoffer <schveiguy@yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |accepts-invalid


--- Comment #4 from Steven Schveighoffer <schveiguy@yahoo.com> 2011-11-10 05:36:46 PST ---
I think this should be accepts-invalid, since the given example code should not compile.

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--- Comment #5 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-10 06:07:48 PST ---
(In reply to comment #4)
> I think this should be accepts-invalid, since the given example code should not compile.

Wow, I'm sorry, and thank you for your fix.

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--- Comment #6 from Steven Schveighoffer <schveiguy@yahoo.com> 2011-11-10 06:49:38 PST ---
(In reply to comment #5)
> (In reply to comment #4)
> > I think this should be accepts-invalid, since the given example code should not compile.
> 
> Wow, I'm sorry, and thank you for your fix.

Don't worry about it!  The main fix is the patch, my change was a nitpick :)

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla@digitalmars.com
         Resolution|                            |FIXED


--- Comment #7 from Walter Bright <bugzilla@digitalmars.com> 2011-11-14 21:11:55 PST ---
https://github.com/D-Programming-Language/dmd/commit/9583a773f8c748ac22872b2fcb5acd3de9bc0186

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