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digitalmars.D.bugs - [Issue 11048] New: Default arguments not taken into account when being called by pure functions

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11048

           Summary: Default arguments not taken into account when being
                    called by pure functions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: jmdavisProg gmx.com


--- Comment #0 from Jonathan M Davis <jmdavisProg gmx.com> 2013-09-15 14:54:15
PDT ---
int x = 7;

void foo() pure
{
    // Does not detect use of mutable global and compiles when it shouldn't.
    bar();

    // Correctly detects the use of a mutable global and gives an error
    baz(x);
}

void bar(int a = x) pure {}

void baz(int a) pure {}

void main() {}

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Sep 15 2013
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11048


Andrej Mitrovic <andrej.mitrovich gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich gmail.com


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich gmail.com> 2013-09-16
08:48:37 PDT ---
So should the declaration of such a function be denied if it's pure, or should
only calls be denied where a global is used? E.g.:

// 1. ban this declaration?
void bar(int a = x) pure {}

// 2. or just this call?
bar();

// If #2, then this could be considered ok:
bar(1);

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Sep 16 2013
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=11048



--- Comment #2 from Jonathan M Davis <jmdavisProg gmx.com> 2013-09-16 09:35:22
PDT ---
The default argument is only a problem if the pure function is called from a
pure function. It's perfectly fine if it's called from an impure one. The
problem is not that the default argument references a global but that the
caller does not detect that the default argument of the function it's calling
is a global. So, the caller does not detect that the call violates purity.

The simplest solution is probably to ban the declaration, because then the
caller doesn't have to worry about whether the pure function that it's calling
has any default arguments which would violate the purity of the caller, but
ideally, it's just the call which would be illegal, because the default
argument is just fine so long as the caller isn't pure.

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