[Bug c++/47783] New: Warning 'set but not used' [-Wunused-but-set-parameter] incorrectly issued for update through reference wrapper

[dev.lists at jessamine dot co.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47783

           Summary: Warning 'set but not used'
                    [-Wunused-but-set-parameter] incorrectly issued for
                    update through reference wrapper
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: dev.li...@jessamine.co.uk


Compiling the following program with warning options -W -Wall yields
<stdin>: In function ‘void f(ref_wrapper)’:
<stdin>:6:6: warning: parameter ‘ref’ set but not used
[-Wunused-but-set-parameter]

If the reference-to-int is replaced with a pointer-to-int and dereference and
address-of operators are used in the appropriate places the warning does not
occur.

In all cases and at all optimization levels the resulting program returns 7 as
expected.


struct ref_wrapper
{
   int& i;
};

void f( ref_wrapper ref )
{
   ref.i = 7;
}

int main()
{
   int x = 1;
   ref_wrapper xref = {x};
   f(xref);
   return x;
}