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

[sgunderson at bigfoot dot com]

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

            Bug ID: 79746
           Summary: Confusing -Wunused-but-set-parameter warning with
                    virtual inheritance
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sgunderson at bigfoot dot com
  Target Milestone: ---

Hi,

This is a minified testcase of MySQL when trying to compile with a 7.0
snapshot:

atum17:~> /usr/lib/gcc-snapshot/bin/g++ -v                                    
Using built-in specs.
COLLECT_GCC=/usr/lib/gcc-snapshot/bin/g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc-snapshot/libexec/gcc/x86_64-linux-gnu/7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 20170226-1'
--with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,ada,c++,go,brig,fortran,objc,obj-c++
--prefix=/usr/lib/gcc-snapshot --with-gcc-major-version-only --program-prefix=
--enable-shared --enable-linker-build-id --disable-nls --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.0.1 20170226 (experimental) [trunk revision 245744] (Debian
20170226-1) 

atum17:~> cat test2.cc
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) : Base(foo) {}
};

atum17:~> /usr/lib/gcc-snapshot/bin/g++ -Wunused-but-set-parameter -c test2.cc
test2.cc: In constructor 'Derived::Derived(const char*)':
test2.cc:9:22: warning: parameter 'foo' set but not used
[-Wunused-but-set-parameter]
  Derived(const char *foo) : Base(foo) {}

I think the warning is actually somehow correct, but it's very confusing until
you see what's going on. I think the logic goes something like: Virtual bases
are always set through the most derived class. Since Derived has a pure virtual
(func()), it can't be the most derived class, and thus, its call to Base() can
never actually happen. Thus, “foo” is unused.

Perhaps a better warning would be something like

test2.cc:9:22: warning: class 'Derived' inherits virtually from 'Base' but is
not possible to instantiate by itself, so it can never be the most derived
class, and the call to 'Base::Base(const char *foo)' is always ignored

but that might be too wordy.