http://d.puremagic.com/issues/show_bug.cgi?id=2807

           Summary: Marking a nested function as 'pure' may cause bad code
                    generations silently accepted
           Product: D
           Version: 2.027
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Keywords: accepts-invalid
          Severity: critical
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: clugdbug@yahoo.com.au


The following assert fails, effectively generating bad code.
My patch in #2804 doesn't fix this, but it does reduce the need for marking
nested functions as pure. A nested function which is marked as pure should not
be able to access variables from the enclosing scope.
It's probably adequate to simply disallow 'pure' on nested functions for now.

---
pure int double_sqr(int x) {
    int y = x;
    void do_sqr() pure { y *= y; }
    do_sqr();
    return y;
}

void main(string[] args) {
   assert(double_sqr(10) == 100);
}


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=2807





------- Comment #1 from clugdbug@yahoo.com.au  2009-04-22 15:07 -------
And here's a patch. This makes it safe to have nested pure functions.
A nested pure function is only permitted to access its own variables, plus
immutables and manifest constants from outer scopes.


===================================================================
--- expression.c        (revision 24)
+++ expression.c        (working copy)
@@ -3994,12 +3994,21 @@
 #if 1
        if (sc->func) {
                FuncDeclaration *outerfunc=sc->func;
+               bool hasPureParent=false;
                while (outerfunc->toParent2() &&
outerfunc->toParent2()->isFuncDeclaration()) {
+                       hasPureParent |= outerfunc->isPure();
                        outerfunc =
outerfunc->toParent2()->isFuncDeclaration();
                }
-    if (outerfunc->isPure()  && !sc->intypeof && v->isDataseg() &&
!v->isInvariant())
+               hasPureParent |= outerfunc->isPure();
+               // If ANY of its enclosing functions are pure, it cannot do
anything impure.
+               // If it is pure, it cannot access any mutable variables other
than those inside itself.
+       if (hasPureParent && !sc->intypeof && v->isDataseg() &&
!v->isInvariant()) {
                error("pure function '%s' cannot access mutable static data
'%s'", sc->func->toChars(), v->toChars());
-       }
+       } else if (sc->func->isPure() && sc->parent!=v->parent && !sc->intypeof
&& !v->isInvariant() && !(v->storage_class & STCmanifest)) {
+                       error("pure nested function '%s' cannot access mutable
data '%s'", sc->func->toChars(), v->toChars());
+                  if (v->isEnumDeclaration()) error("enum");
+       }
+       }
 #else
        if (sc->func && sc->func->isPure() && !sc->intypeof)
        {


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=2807





------- Comment #2 from clugdbug@yahoo.com.au  2009-04-22 15:09 -------
Created an attachment (id=335)
 --> (http://d.puremagic.com/issues/attachment.cgi?id=335&action=view)
Patch against 2.029. Includes patch for the related bug 2695, as well.


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=2807


Don <clugdbug@yahoo.com.au> changed:

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




--- Comment #3 from Don <clugdbug@yahoo.com.au>  2009-05-14 01:27:15 PDT ---
Fixed DMD2.030

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