[Bug c++/39551] New: C++ frontend not warn about unused dereference operator with -Wunused-value

[lcwu at gcc dot gnu dot org]

Given the following code, where the value of the dereference operation is
unused, 

$ cat unused_deref.c
void Foo(int* x) {
  *x++;
}

if it is compiled with gcc (C frontend) with -Wunused-value option, we get a
warning.

$ gcc -Wunused-value -c unused_deref.c
unused_deref.c: In function 'Foo':
unused_deref.c:2: warning: value computed is not used

However, if the code is compiled with g++, we don't get the warning.

$ g++ -Wunused-value -c unused_deref.c


After doing some triage, it appears that C++ frontend silently gets rid of
INDIRECT_REF operator (without emitting any warnings) when it processes the
expression '*x++' in convert_to_void() in cp/cvt.c.

Here is the version string of the compiler that I tried:
gcc version 4.4.0 20090324 (experimental) (GCC)


-- 
           Summary: C++ frontend not warn about unused dereference operator
                    with -Wunused-value
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: lcwu at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39551