libgc bug when using Gnu C nested functions

[Glyn]

/*
This small program causes a libgc error when compiled and run in Cygwin:

     $ gcc gcerror.c -lgc -o x.exe
     $ ./x.exe
     42
     GC Warning: Thread stack pointer 0x22aa98 out of range, pushing 
everything
*/

#include <gc/gc.h>
#include <stdio.h>

void *dummy;

void f (void (*x)())
{
     x();
}

int main ()
{
     int i;

     void g ()
     {
         printf("%i\n", i);
     }

     i = 42;
     f(g);
     dummy = GC_MALLOC(10);

     return 0;
}

/*
The presence of a function call that is passed a pointer to
a Gnu C nested function with a closure seems to cause the problem:

     * There is no error if:

         - i is global;
         - i is not used in g;
         - g is called normally (not passed to f);
         - f is not called;
         - GC_MALLOC is not called;
         - GC_MALLOC is replaced with malloc.

     * There is still an error if:

         - f does not call g;
         - GC_MALLOC is called before f.

The bug also causes memory corruption when it occurs larger programs.

Programs with libgc and this construct always work correctly in Debian 
Stable.
*/

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