Implement P2280R4 Using unknown pointers and references in constant expressions

[cor3ntin]

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-2b

[KyunLFA]

Valve's Gamescope on git HEAD does not build anymore due to this error if my (limited) understanding is correct.

GCC passes, Clang doesn't.

FAILED: src/gamescope.p/wayland_backend.cpp.o 
/usr/bin/clang++ -Isrc/gamescope.p -Isrc -I../gamescope/src -I../gamescope/src/reshade/source -I../gamescope/src/reshade/include -I../gamescope/thirdparty/SPIRV-Headers/include/spirv/unified1 -Isubprojects/glm -I../gamescope/subprojects/glm -Isubprojects/reshade -I../gamescope/subprojects/reshade -Isubprojects/wlroots/include -I../gamescope/subprojects/wlroots/include -I../gamescope/subprojects/libdisplay-info/include -I../gamescope/subprojects/libliftoff/include -Isubprojects/stb -I../gamescope/subprojects/stb -Iprotocol -I/usr/include/libdrm -I/usr/include/SDL2 -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/pipewire-0.3 -I/usr/include/spa-0.2 -I/usr/include/openvr -I/usr/include/libdecor-0 -fdiagnostics-color=always -D_GLIBCXX_ASSERTIONS=1 -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -std=c++20 -DWLR_USE_UNSTABLE -Wno-unused-parameter -Wno-missing-field-initializers -Wno-c99-designator -Wno-invalid-offsetof -Wno-unused-const-variable -Wno-deprecated-volatile -Wno-ignored-qualifiers -Wno-missing-braces -ffast-math -DHAVE_PIPEWIRE=1 -DHAVE_OPENVR=1 '-DHWDATA_PNP_IDS="//usr/share/hwdata/pnp.ids"' -flto=full -march=znver4 -O3 -pipe -fno-plt -fexceptions -Wp,-D_FORTIFY_SOURCE=3 -Wformat -Werror=format-security -fstack-protector-all -fcf-protection -fno-semantic-interposition -Wno-unused-command-line-argument -mllvm -polly -fwhole-program-vtables -Wp,-D_GLIBCXX_ASSERTIONS -fPIE -DAVIF_DLL -isystem/usr/include/libdrm -D_REENTRANT -pthread -DHAVE_DRM=1 -DHAVE_SDL2=1 -DHAVE_AVIF=1 -DHAVE_LIBCAP=1 -MD -MQ src/gamescope.p/wayland_backend.cpp.o -MF src/gamescope.p/wayland_backend.cpp.o.d -o src/gamescope.p/wayland_backend.cpp.o -c ../gamescope/src/wayland_backend.cpp
In file included from ../gamescope/src/wayland_backend.cpp:1:
In file included from ../gamescope/src/backend.h:7:
../gamescope/src/rc.h:171:36: error: constexpr if condition is not a constant expression
  171 |                     if constexpr ( m_pObject->ReferenceOwnsObject() )
      |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../gamescope/src/rc.h:140:19: note: in instantiation of member function 'gamescope::Rc<gamescope::CWaylandFb>::DecRef' requested here
  140 |             this->DecRef();
      |                   ^
../gamescope/src/wayland_backend.cpp:1111:22: note: in instantiation of member function 'gamescope::Rc<gamescope::CWaylandFb>::~Rc' requested here
 1111 |     CWaylandBackend::CWaylandBackend()
      |                      ^
../gamescope/src/rc.h:171:36: note: implicit use of 'this' pointer is only allowed within the evaluation of a call to a 'constexpr' member function
  171 |                     if constexpr ( m_pObject->ReferenceOwnsObject() )
      |                                    ^
../gamescope/src/rc.h:171:36: error: constexpr if condition is not a constant expression
  171 |                     if constexpr ( m_pObject->ReferenceOwnsObject() )
      |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../gamescope/src/rc.h:140:19: note: in instantiation of member function 'gamescope::Rc<gamescope::IBackendFb>::DecRef' requested here
  140 |             this->DecRef();
      |                   ^
../gamescope/src/wayland_backend.cpp:1231:69: note: in instantiation of member function 'gamescope::Rc<gamescope::IBackendFb>::~Rc' requested here
 1231 |             m_BlackFb = static_cast<CWaylandFb *>( m_pBlackTexture->GetBackendFb().get() );
      |                                                                     ^
../gamescope/src/rc.h:171:36: note: implicit use of 'this' pointer is only allowed within the evaluation of a call to a 'constexpr' member function
  171 |                     if constexpr ( m_pObject->ReferenceOwnsObject() )
      |                                    ^
2 errors generated.

Full context: https://github.com/ValveSoftware/gamescope/blob/master/src/rc.h

Tested on CachyOS (Arch) Linux, LLVM 19.0.0 1c683eb. (Workaround suggestions (besides switching compilers) appreciated)