warn_unused_result attribute and returning class o

Question

Please consider the following code:

class cls
{
};

__attribute__((warn_unused_result))
cls fn1()
{
    return cls();
}

__attribute__ ((warn_unused_result))
int fn2()
{
    return 1;
}


int main()
{
    auto x = fn1();
    (void)x;

    auto y = fn2();
    (void)y;

    return 0;
}

When I compile this with -Wall -Wextra with gcc 5.4.0, I get a warning:

warning: ignoring return value of ‘cls fn1()’, 
declared with attribute warn_unused_result [-Wunused-result]

Can somebody tell me what's wrong?
Why this warning appears for fn1 despite the x variable, and why it does NOT appear for the fn2?

---

Update 1: With a little help, I tried this with g++ 4.8.3 - the same result. Even more - g++ 3.3.6 produces two warnings - for fn1 and fn2.

What "fixes" the warning is...... guess what - adding a data member or a virtual member function in the class (a non-virtual function does not help). Sounds like a compiler bug to me.

Update 2: Actually adding a member or a virtual function disables the warning completely, so just calling fn1(); does NOT produce the warning anymore.