https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123955
Bug ID: 123955
Summary: ‘Reference to void’ error for C++20 partial
specialization of class template with void
Product: gcc
Version: 13.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: htamakrg at gmail dot com
Target Milestone: ---
Created attachment 63576
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=63576&action=edit
Preprocessed source file of minimal example
Component:
C++ front end
GCC version:
Using built-in specs.
COLLECT_GCC=/usr/bin/c++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/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
13.3.0-6ubuntu2~24.04' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --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-libstdcxx-backtrace
--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-13-fG75Ri/gcc-13-13.3.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-fG75Ri/gcc-13-13.3.0/debian/tmp-gcn/usr
--enable-offload-defaulted --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 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04)
System:
x86_64-linux-gnu
Command line:
/usr/bin/c++ -D_GLIBCXX_DEBUG -g -O0 -std=c++20 -Wall -Wextra -Wpedantic
-Werror -fno-strict-aliasing -fwrapv -fsanitize=address,undefined
-fno-omit-frame-pointer -o main.cc.o -c main.cc
Compiler output:
main.cc: In function ‘int main()’:
main.cc:25:29: error: forming reference to void
25 | InvariantClass<int, void> iv{};
| ^~
Description:
When partially specialising a class with void and then instantiating said
class, void is substituted in std::copyable and gives error with error message
“reference to void”. This program is accepted by Clang 16 (and higher; verified
till Clang 21.1.0) but rejected by GCC 13.3 (and higher; fails even in GCC
15.2). The attached preprocessed file (only the code snippet at the end is
needed) reproduces the issue.
Workaround:
Scoping all concepts in a requires block to delay substitution.
```cpp
template <typename T, typename CloseStatus>
concept InvariantClassLike = requires {
requires std::movable<T> &&
(std::same_as<CloseStatus, void> || std::copyable<CloseStatus>);
};
```
