[Bug c++/79746] Confusing -Wunused-but-set-parameter warning with virtual inheritance

[sgunderson at bigfoot dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79746

--- Comment #1 from sgunderson at bigfoot dot com ---
Actually this is interesting; this code (derived from the previous one)
compiles without warning in GCC 7.0 and Clang, but gives an error in GCC 6.3:

struct Base {         
        Base(const char *foo) : m_foo(foo) {}
        virtual int func() = 0;

        const char *m_foo;
};

struct Derived : public virtual Base {
        Derived(const char *foo) { (void)foo; }
};

Which compiler is right?