http://d.puremagic.com/issues/show_bug.cgi?id=5314

           Summary: Wrong error message: struct within immutable member
                    and postblit function
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: rayerd.wiz@gmail.com


--- Comment #0 from Haruki Shigemori <rayerd.wiz@gmail.com> 2010-12-03 09:19:34 PST ---
// a.d
struct ID
{
        immutable int value;
        this(this){}
}
void main() {}

$ dmd a.d
Error: this is not mutable
Error: this is not mutable
Error: this is not mutable

- No line number.
- Strange *three* error message.

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Jonathan M Davis <jmdavisProg@gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jmdavisProg@gmx.com


--- Comment #1 from Jonathan M Davis <jmdavisProg@gmx.com> 2010-12-03 11:37:52 PST ---
I've noticed that a lot of errors get printed three times. I have no idea why. Regardless, the error message is close to correct. The problem is, of course, that value is immutable, not the this pointer, but the object as a whole is not mutable, because value is immutable, so saying that this is not mutable is essentially correct, if somewhat misleading. It needs a better error message, but the one that's there isn't totally off.

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--- Comment #2 from Haruki Shigemori <rayerd.wiz@gmail.com> 2010-12-03 18:44:54 PST ---
(In reply to comment #1)
> I've noticed that a lot of errors get printed three times. I have no idea why. Regardless, the error message is close to correct. The problem is, of course, that value is immutable, not the this pointer, but the object as a whole is not mutable, because value is immutable, so saying that this is not mutable is essentially correct, if somewhat misleading. It needs a better error message, but the one that's there isn't totally off.

Of course, I know that.
I said that the error message has two problems.
I didn't say that it is unkindness.
However, it makes me happy if the message is more better, because the current
message has not class and variable name.

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nfxjfg@gmail.com changed:

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


--- Comment #3 from nfxjfg@gmail.com 2010-12-03 19:48:37 PST ---
Are you saying this is not a rejects-valid?

I fail to see why structs with immutable members shouldn't be allowed to have a postblit function.

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--- Comment #4 from Haruki Shigemori <rayerd.wiz@gmail.com> 2010-12-04 00:16:09 PST ---
(In reply to comment #3)
> Are you saying this is not a rejects-valid?

I think that this is maybe a rejects-invalid correctly.

Error: this is not mutable
Error: this is not mutable
Error: this is not mutable

However, this error message is bad clearly, because it hasn't line numbers and
type/variable names.
Of course, this message is strange that it is written by three times.

For example, I think that a following sentence is more better.

filename.d(4): struct ID has not both postblit and immutable member value

> I fail to see why structs with immutable members shouldn't be allowed to have a postblit function.

Uhmmm... I don't discuss the matter because it is other problem. But, we need probably discussion of the problem that you said.

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--- Comment #5 from Haruki Shigemori <rayerd.wiz@gmail.com> 2010-12-04 00:18:22 PST ---
- filename.d(4): struct ID has not both postblit and immutable member value
+ filename.d(4): struct ID cannot have both postblit and immutable member value

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--- Comment #6 from Jonathan M Davis <jmdavisProg@gmx.com> 2010-12-06 00:21:15 PST ---
@nfxjfg

This program shows when init, a constructor, the postblit constructor, and opAssign are used:

import std.stdio;

struct Foo
{
    int a = 7;

    this(int a)
    {
        this.a = a;
        writefln("constructor: %s", this.a);
    }

    this(this)
    {
        writefln("postblit: %s", this.a);
    }

/+
    Foo opAssign(const ref Foo other)
    {
        this.a = other.a;
        writefln("opAssign: %s", this.a);

        return this;
    }
+/
}

void main()
{
    writefln("%s(%s): -------", __FILE__, __LINE__);
    Foo a;
    writefln("%s(%s): -------", __FILE__, __LINE__);
    auto b = Foo(6);
    writefln("%s(%s): -------", __FILE__, __LINE__);
    auto c = a;
    writefln("%s(%s): -------", __FILE__, __LINE__);
    b = c;
    writefln("%s(%s): -------", __FILE__, __LINE__);
}


The output is:

test.d(29): -------
test.d(31): -------
constructor: 6
test.d(33): -------
postblit: 7
test.d(35): -------
postblit: 7
test.d(37): -------


Notice that assignment uses the postblit constructor. If you uncomment opAssign(), then you get this instead:

test.d(29): -------
test.d(31): -------
constructor: 6
test.d(33): -------
postblit: 7
test.d(35): -------
opAssign: 7
test.d(37): -------


Now, assignment uses opAssign(). The problem with having an immutable member variable and a postblit constructor is that you cannot reassign that member variable. That portion of the struct cannot be altered.

If I changed the code to

import std.stdio;

struct Foo
{
    immutable int a;

    this(int a) immutable
    {
        this.a = a;
        writefln("constructor: %s", this.a);
    }

}

void main()
{
    writefln("%s(%s): -------", __FILE__, __LINE__);
    Foo a;
    writefln("%s(%s): -------", __FILE__, __LINE__);
    auto b = Foo(6);
    writefln("%s(%s): -------", __FILE__, __LINE__);
    auto c = a;
    writefln("%s(%s): -------", __FILE__, __LINE__);
    b = c;
    writefln("%s(%s): -------", __FILE__, __LINE__);
}


then it fails to compile, giving the message

test.d(24): Error: variable test.main.b cannot modify immutable


The line b = c is illegal. Now, the line auto c = a is still legal, and you could arguably allow a postblit constructor which didn't alter the immutable member variables, but it does get a bit odd. You could arguably even allow for assignment via opAssign() as long as the immutable member variable isn't altered, but then it's probably not really copying, which would not be good.

So, I suppose that a postblit could be allowed in a struct with an immutable member variable, but you couldn't assign to it or do a deep copy or do anything with it that you'd normally do in a postblit constructor (though whether or not that's a problem is debatable). But the compiler would have to be smart enough to realize that having a postblit constructor didn't allow for assignment for structs with immutable member variables, which may or may not be straightforward, depending on how the compiler decides that it cannot do an assignment to a struct with an immutable member variable.

All in all, copying structs with immutable member variables does tend to get a bit funny. I suppose that a postblit constructor _could_ be allowed, but it is a bit odd. Assignment certainly can't be allowed, and the postblit constructor is assignment of a sort, but it could still work as long as it's only used when constructing a variable. So, I don't know what the correct behavior is.

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kennytm@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ultimatemacfanatic@gmail.co
                   |                            |m


--- Comment #7 from kennytm@gmail.com 2011-05-01 23:32:49 PDT ---
*** Issue 5917 has been marked as a duplicate of this issue. ***

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kennytm@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
                 CC|                            |kennytm@gmail.com
         OS/Version|Windows                     |All
           Severity|normal                      |critical


--- Comment #8 from kennytm@gmail.com 2011-05-01 23:35:22 PDT ---
Raising severity to "Critical" as per bug 5745.

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SomeDude <lovelydear@mailmetrash.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lovelydear@mailmetrash.com
           Severity|critical                    |normal


--- Comment #9 from SomeDude <lovelydear@mailmetrash.com> 2012-05-04 22:48:45 PDT ---
Reducing criticity to normal, as diagnostic bugs are not critical.

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