http://d.puremagic.com/issues/show_bug.cgi?id=6922

           Summary: [TDPL] superimposing of const and immutable does not
                    work correctly
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: timon.gehr@gmx.ch


--- Comment #0 from timon.gehr@gmx.ch 2011-11-09 14:23:36 PST ---
From TDPL, p. 299:
"When two qualifiers are superimposed, D uses simple composition rules. If the
qualifiers are identical, they are collapsed into one. Otherwise,
const(immutable(T)) and immutable(const(T)) are both collapsed into
immutable(T) because that is the most informative type."


However, with DMD 2.056:

static assert(is(immutable(const(T)) == immutable(T)));        // fail.
static assert(is(const(immutable(T)) == immutable(const(T)))); // fail.
static assert(is(const(immutable(T)) == immutable(T)));        // fail.

All three assertions should pass.

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla@kyllingen.net


--- Comment #1 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-10 01:42:21 PST ---
*** Issue 6338 has been marked as a duplicate of this issue. ***

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch, rejects-valid


--- Comment #2 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-10 01:42:51 PST ---
https://github.com/D-Programming-Language/dmd/pull/505

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--- Comment #3 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-10 12:48:18 PST ---
From discussion in https://github.com/D-Programming-Language/dmd/pull/505

1. inout + const of T should parse [1a] const(T) or [1b] inout(T)?
2. or introduce new combined qualifier inout(const(T))? (see bug 6930)

Current my patch have selected [1a] as the result.

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--- Comment #4 from timon.gehr@gmx.ch 2011-11-10 12:57:55 PST ---
(In reply to comment #3)
> From discussion in https://github.com/D-Programming-Language/dmd/pull/505
> 
> 1. inout + const of T should parse [1a] const(T) or [1b] inout(T)?
> 2. or introduce new combined qualifier inout(const(T))? (see bug 6930)
> 
> Current my patch have selected [1a] as the result.


Parsing it as inout(T) or parsing it as const(T) is both wrong (and any of the
two are an arbitrary choice).

The 'definition' of inout is that it is whatever qualifier is on the input argument.

Consider the type

inout(const(T))

We can now make a case distinction:

1. inout == mutable   => inout(const(T)) == const(T)
2. inout == const     => inout(const(T)) == const(const(T)) == const(T)
3. inout == immutable => inout(const(T)) == immutable(const(T)) == immutable(T)

Option 1. does not support the 3rd case, therefore it is wrong. Option 2. works as expected, therefore it is correct.

I do not think there is a choice. (adding unnecessary special cases is the way to make a language ugly and complicated, see C++)

If you disagree, with what part of the explanation do you disagree?

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--- Comment #5 from Kenji Hara <k.hara.pg@gmail.com> 2011-11-10 13:10:16 PST ---
(In reply to comment #4)
> If you disagree, with what part of the explanation do you disagree?

No, I don't disagree your explanation. My only argument is that is *debatable* thing.

OK. I'll change my patch to make the combination of inout and const ambiguous (== keep current behavior) in order to make room for improvement.

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--- Comment #6 from timon.gehr@gmx.ch 2011-11-10 13:14:59 PST ---
(In reply to comment #5)
> (In reply to comment #4)
> > If you disagree, with what part of the explanation do you disagree?
> 
> No, I don't disagree your explanation. My only argument is that is *debatable* thing.

Apparently it is. :o)

> 
> OK. I'll change my patch to make the combination of inout and const ambiguous (== keep current behavior) in order to make room for improvement.

Thank you.

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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 20:04:36 PST ---
https://github.com/D-Programming-Language/dmd/commit/b8ec4cf0e44ca069de5df33bde37639ab8281c8d

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--- Comment #8 from Kenji Hara <k.hara.pg@gmail.com> 2011-12-11 19:34:35 PST ---
*** Issue 6855 has been marked as a duplicate of this issue. ***

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