[Bug c++/97230] New: Invocation of non-static member function on a null instance in core constant expression should not be allowed

[Bug c++/97230] New: Invocation of non-static member function on a null instance in core constant expression should not be allowed

[eligorkadaf at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97230

            Bug ID: 97230
           Summary: Invocation of non-static member function on a null
                    instance in core constant expression should not be
                    allowed
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eligorkadaf at gmail dot com
  Target Milestone: ---

The code below should not compile according to the C++ standart.

The code:
class X{

public:
    constexpr int foo(){
        return 1;
    }
};

int main(){
    constexpr auto x = ((X*)nullptr)->foo();
}

Compile and run logs:

$ g++-10 -v
Using built-in specs.
COLLECT_GCC=g++-10
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10-20200411-0ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--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-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 --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none,amdgcn-amdhsa,hsa --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.0.1 20200411 (experimental) [master revision
bb87d5cc77d:75961caccb7:f883c46b4877f637e0fa5025b4d6b5c9040ec566] (Ubuntu
10-20200411-0ubuntu1)

$ g++-10 -Wall -Wextra -std=c++20 bug.cpp
bug.cpp: In function ‘int main()’:
bug.cpp:10:20: warning: unused variable ‘x’ [-Wunused-variable]
   10 |     constexpr auto x = ((X*)nullptr)->foo();
      |                    ^

There are additional non-issue warnings whether the code is compiled by gcc
trunk

The godbolt link: https://godbolt.org/z/rx3axf

The problem is reproducible with gcc 4.8.1, gcc 5.1 and gcc 8.1 also

[Bug c++/97230] Invocation of non-static member function on a null instance in core constant expression should not be allowed

[eligorkadaf at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97230

--- Comment #1 from Eligor Kadaf <eligorkadaf at gmail dot com> ---
Basically it described in 7th paragraph in [expr.const] of the latest C++
standard draft: https://eel.is/c++draft/expr.const#5

[Bug c++/97230] Invocation of non-static member function on a null instance in core constant expression should not be allowed

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97230

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-09-28
     Ever confirmed|0                           |1
           Keywords|compile-time-hog            |accepts-invalid

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC trunk does warn about exactly the issue:

ce.cc: In function 'int main()':
ce.cc:10:43: warning: 'this' pointer null [-Wnonnull]
   10 |     constexpr auto x = ((X*)nullptr)->foo();
      |                                           ^
ce.cc:4:19: note: in a call to non-static member function 'constexpr int
X::foo() const'
    4 |     constexpr int foo(){
      |                   ^~~


But that should be an error during constant evaluation.

[Bug c++/97230] Invocation of non-static member function on a null instance in core constant expression should not be allowed

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97230

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
                 CC|                            |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/97230] Invocation of non-static member function on a null instance in core constant expression should not be allowed

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97230

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
https://gcc.gnu.org/pipermail/gcc-patches/2020-October/555282.html