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digitalmars.D.bugs - [Issue 7968] New: Failing implicit cast, new in 2.059

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7968

           Summary: Failing implicit cast, new in 2.059
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: turkeyman gmail.com


--- Comment #0 from Manu <turkeyman gmail.com> 2012-04-22 11:24:21 PDT ---
This just appeared in 2.059:

void failCast(const(char)*[] arg)
{
}

void test(char** ppStr, uint numStrings)
{
    failCast(ppStr[0..numStrings]);
}


SQLiteDB.d(21): Error: function demu.tools.sqlitedb.failCall (const(char)*[]
arg) is not callable using argument types (char*[])
SQLiteDB.d(21): Error: cannot implicitly convert expression
(ppStr[0u..numStrings]) of type char*[] to const(char)*[]

The chaps on IRC seemed to think this was a bug and I should report it.

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next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=7968


timon.gehr gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |timon.gehr gmx.ch
         Resolution|                            |INVALID


--- Comment #1 from timon.gehr gmx.ch 2012-04-22 11:51:45 PDT ---
(In reply to comment #0)

 The chaps on IRC seemed to think this was a bug and I should report it.


They were wrong.

void failCast(const(char)*[] arg) {
    arg[0] = "123".ptr;
}

void main() {
    char*[] x = ["234".dup.ptr];
    failCast(x); // if this passes then x[0] has type char*
    assert(x[0][0..3] == "123");  //  and points into the immutable data
segment
}

Change the signature of failCast to

void failCast(inout(char)*[] arg)

to get the desired semantics.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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--- Comment #2 from Manu <turkeyman gmail.com> 2012-04-22 17:02:14 PDT ---
(In reply to comment #1)

 (In reply to comment #0)

 The chaps on IRC seemed to think this was a bug and I should report it.


 
 They were wrong.
 
 void failCast(const(char)*[] arg) {
     arg[0] = "123".ptr;
 }
 
 void main() {
     char*[] x = ["234".dup.ptr];
     failCast(x); // if this passes then x[0] has type char*
     assert(x[0][0..3] == "123");  //  and points into the immutable data
 segment
 }
 
 Change the signature of failCast to
 
 void failCast(inout(char)*[] arg)
 
 to get the desired semantics.


I have already changed it so that it works similarly to how you describe, but I
still don't follow why the cast isn't valid?
How does the cast I attempted break the const model?

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--- Comment #3 from timon.gehr gmx.ch 2012-04-23 10:57:19 PDT ---
(In reply to comment #2)

 How does the cast I attempted break the const model?


Having it be valid code allows to get a mutable reference to immutable data.
This reference can then be used to (attempt to) modify the immutable data. You
can refer to my first post for the counter-example.

It is an instance of the more general rule that mutable field arrays cannot be
covariant and type safe without runtime checks on every store to (or load from)
one of their array fields.

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