[Bug c++/66177] New: warn_unused_result doesn't always work

["gcc at bobbyperu dot info" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66177

            Bug ID: 66177
           Summary: warn_unused_result doesn't always work
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at bobbyperu dot info
  Target Milestone: ---

Testcase fails to work with gcc, works with clang.

=====
struct QSize
{
    QSize(int w, int h) : wd(w), ht(h) {}

    QSize expandedTo() const __attribute__ ((__warn_unused_result__))
    {
        return QSize(2, 3);
    }

private:
    int wd;
    int ht;
};

void foo()
{
    QSize sz(2, 2);
    sz.expandedTo();
}
=====
$ g++ testcase.cpp -c -Wunused-result

$ clang++ testcase.cpp -c -Wunused-result
testcase.cpp:18:5: warning: ignoring return value of function declared with
warn_unused_result attribute [-Wunused-result]
    sz.expandedTo();
    ^~~~~~~~~~~~~
1 warning generated.