[Bug c++/15083] New: spurious "statement has no effect" warning

[carlton at bactrian dot org]

When I compile the following program with g++ -Wall (where g++ is the
released 3.4.0), I get the following warning:

quartette$ /net/quartette/carlton/gcc/gcc-3.4.0/install/bin/g++ -Wall test.cpp
test.cpp: In member function `void Foo<T>::resetData()':
test.cpp:31: warning: statement has no effect

The line in question is this one:

        new ( refLoc ) Counter();

but Counter's contructor does have a side effect.  (Indeed, the output
of the program changes if you compile out that line.)

Here's the source code; I'll attach a .ii file as well.  (The .ii
file is kind of large, because I include <iostream> to show that
the line in question does indeed change the behavior of the program.)

#include <new>
#include <iostream>

class Counter {
public:
    void decrement() { --ref_; }

    Counter() : ref_(1) {}

    int ref() const {
        return ref_;
    }

private:
    int ref_;
};

template< typename T >
class Foo {
  public:
    void resetData() {
        ref_.decrement();
        std::cout << "before: " << ref_.ref() << std::endl;
        ref_.~Counter();
        void *refLoc = &ref_;

        // GCC erroneously claims that the following line has no
        // effect.  But it does have an effect: the value of the
        // "after" message will be 0 instead of 1 if you comment it
        // out.
        new ( refLoc ) Counter();

        std::cout << "after: " << ref_.ref() << std::endl;
    }

  private:
    Counter ref_;
};

int main() {
    Foo< int > node;

    node.resetData();
}

-- 
           Summary: spurious "statement has no effect" warning
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carlton at bactrian dot org
                CC: gcc-bugs 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=15083