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digitalmars.D.bugs - [Issue 9230] New: Incorrect implicit immutable conversion occurs in pure function

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9230

           Summary: Incorrect implicit immutable conversion occurs in pure
                    function
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: accepts-invalid
          Severity: regression
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: k.hara.pg gmail.com



From: http://d.puremagic.com/issues/show_bug.cgi?id=6553

In git head, following code compiles without errors, and asserts in runtime.

string foo(in char[] s) pure {
    return s; // conversion from const(char[]) to string
}
void main() {
    char[] buf = ['a'];
    immutable str = foo(buf);
    assert(str[0] == 'a');
    buf[0] = 'b';
    assert(str[0] == 'a');  // fails!?
}

There is a problem on the return statement in foo. Complier deduces foo to a
strong purity function, but its parameter s might refer outer mutable state, so
the conversion const(char[]) to string should not be applied.

This is an implementation bug of issue 5081.

https://github.com/D-Programming-Language/dmd/commit/99e0f21d2a7a19b655d97fa08394a3bc623f10a0

https://github.com/D-Programming-Language/dmd/commit/99e0f21d2a7a19b655d97fa08394a3bc623f10a0#L3R3524

The implicit immutable conversion on return statement is valid only if the
strong purity function has *no parameters*.

This bug has appeared by the purity calculation improvement by issue 8408.

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next sibling parent d-bugmail puremagic.com writes:
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 The implicit immutable conversion on return statement is valid only if the
 strong purity function has *no parameters*.


...No, this is incorrect. I found a disproof.

Correct requirement is:
"If all parameters have no mutable indirections, implicit immutable conversion
on return statement is allowed."

For example:

immutable(int[]) foo(int n) { return new int[](n); }
immutable(int[]) bar(immutable int[] arr) { return new int[](arr.length); }
void main() {
    immutable a = foo(10);
    immutable b = bar([1,2]);
}

Both foo and bar should be allowed.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull



https://github.com/D-Programming-Language/dmd/pull/1418

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yebblies <yebblies gmail.com> changed:

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





 From: http://d.puremagic.com/issues/show_bug.cgi?id=6553
 
 In git head, following code compiles without errors, and asserts in runtime.
 
 string foo(in char[] s) pure {
     return s; // conversion from const(char[]) to string
 }
 void main() {
     char[] buf = ['a'];
     immutable str = foo(buf);
     assert(str[0] == 'a');
     buf[0] = 'b';
     assert(str[0] == 'a');  // fails!?
 }
 
 There is a problem on the return statement in foo. Complier deduces foo to a
 strong purity function, but its parameter s might refer outer mutable state, so
 the conversion const(char[]) to string should not be applied.
 
 This is an implementation bug of issue 5081.
 
 This bug has appeared by the purity calculation improvement by issue 8408.


The patch for issue 5081 was corrent, the problem is that the definition of
strong purity has been changed since then.





 The implicit immutable conversion on return statement is valid only if the
 strong purity function has *no parameters*.


 
 ...No, this is incorrect. I found a disproof.
 
 Correct requirement is:
 "If all parameters have no mutable indirections, implicit immutable conversion
 on return statement is allowed."
 


This is the old definition of strong purity.

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 Correct requirement is:
 "If all parameters have no mutable indirections, implicit immutable conversion
 on return statement is allowed."
 


Maybe it should say 'no non-immutable indirections'?

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 The patch for issue 5081 was corrent, the problem is that the definition of
 strong purity has been changed since then.



 This is the old definition of strong purity.


Hmm...interesting, and it might be true. I have thought that "the old
definition" was a conservative rule, and did proposed issue 8408 to improve it.
But, based on your talk, I have *changed* the definition of strong purity more
strictly.

I withdraw the claim that it was an implementation bug.





 Correct requirement is:
 "If all parameters have no mutable indirections, implicit immutable conversion
 on return statement is allowed."


 
 Maybe it should say 'no non-immutable indirections'?


Yes, it is more exact explanation.
(I sometimes called the const qualified indirections to "mutable indirection" 
- e.g. const(int)[]. Because it is an opposite of "immutable indirection".)

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Commits pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/afa7a54422dd3ee7d6622e0b83c5c56c238023fb
fix Issue 9230 - Incorrect implicit immutable conversion occurs in pure
function

`hasMutableIndirectionParams()` is based on the old
`TypeFunction::purityLevel()` (before fixing issue 8408). So its result is
consistent with `TypeFunction::purityLevel() != PUREstrong` in 2.060. Then it
*fixes* the regression.

https://github.com/D-Programming-Language/dmd/commit/c42d35bcf92cd26c2b83c3d2e5fc21e5a0f2d296


Issue 9230 - Incorrect implicit immutable conversion occurs in pure function

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Kenji Hara <k.hara.pg gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


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Commit pushed to staging at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/260ec594ad14acf72812c2563ab249b71a7c55d2


Issue 9230 - Incorrect implicit immutable conversion occurs in pure function

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prev sibling parent d-bugmail puremagic.com writes:
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Issue 8998 is related

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Jan 16 2013