[Bug c++/109761] New: Nested class destructor's noexcept specification incorrectly considered as too loose compared to the outer class

["andysem at mail dot ru" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109761

            Bug ID: 109761
           Summary: Nested class destructor's noexcept specification
                    incorrectly considered as too loose compared to the
                    outer class
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andysem at mail dot ru
  Target Milestone: ---

Consider the following piece of code:

template< typename T >
T declval() noexcept;

struct base
{
    virtual ~base() {}
};

class outer :
    public base
{
protected:
    class nested
    {
    private:
        outer& m_outer;

    public:
        explicit nested(outer& o) noexcept(noexcept(o.on_nested_ctor())) :
            m_outer(o)
        {
            o.on_nested_ctor();
        }

        nested(nested const&) = delete;
        nested& operator= (nested const&) = delete;

        ~nested() noexcept(noexcept(declval<outer&>().on_nested_dtor()))
        {
            m_outer.on_nested_dtor();
        }
    };

public:
    ~outer() {}

protected:
    void on_nested_ctor();
    void on_nested_dtor();
};

This wrongly fails to compile:

$ g++ -std=gnu++17 -o nested_noexcept_dtor.o -c nested_noexcept_dtor.cpp 
nested_noexcept_dtor.cpp:28:9: error: looser exception specification on
overriding virtual function ‘outer::nested::~nested() noexcept (false)’
   28 |         ~nested()
noexcept(noexcept(declval<outer&>().on_nested_dtor()))
      |         ^
nested_noexcept_dtor.cpp:6:13: note: overridden function is ‘virtual
base::~base() noexcept’
    6 |     virtual ~base() {}
      |             ^

The compiler tries to match the ~nested() noexcept specification against the
~base()'s as if nested was derived from base, but it is not. ~nested() noexcept
specification has no bearing on the outer class hierarchy.

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.3.0-1ubuntu1~22.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-xKiWfi/gcc-11-11.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-xKiWfi/gcc-11-11.3.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.3.0 (Ubuntu 11.3.0-1ubuntu1~22.04)