Can g++ (gcc) 11 be used on Debian Bullseye?

Here it is, g++-11 from the default Ubuntu repositories ready to install in Debian Bullseye and the package's dependencies are listed on this page.

Open the terminal and type:

cd ~/Desktop/
wget http://security.ubuntu.com/ubuntu/pool/universe/g/gcc-11/g++-11_11.1.0-1ubuntu1~21.04_amd64.deb
apt install --simulate ./g++-11_11.1.0-1ubuntu1~21.04_amd64.deb

The last command will return a list of missing dependencies which can be found at this webpage. After all the missing dependency packages have been manually downloaded and installed, the command to install the g++-11 package is as follows:

sudo apt install ./g++-11_11.1.0-1ubuntu1~21.04_amd64.deb 

The apt install --simulate commands will certainly generate unmet dependencies errors at first because the Debian Bullseye default repositories do not contain the gcc-11 dependency package (gcc-10 is the latest version), but they will show a list of missing dependency packages. If you work at it iteratively you will be able to get all of these missing dependency packages installed, so that in the end sudo apt install ./g++-11_11.1.0-1ubuntu1~21.04_amd64.deb will run without errors.

g++-11, gcc-11 installs OK into 'Debian 11 Bullseye'.

g++-11 and dependencies ...

cpp-11_11.1.0-4_amd64.deb
g++-11_11.1.0-4_amd64.deb
gcc-11_11.1.0-4_amd64.deb
gcc-11-base_11.1.0-4_amd64.deb
libasan6_11.1.0-4_amd64.deb
libatomic1_11.1.0-4_amd64.deb
libcc1-0_11.1.0-4_amd64.deb
libgcc-11-dev_11.1.0-4_amd64.deb
libgcc-s1_11.1.0-4_amd64.deb
libgomp1_11.1.0-4_amd64.deb
libitm1_11.1.0-4_amd64.deb
liblsan0_11.1.0-4_amd64.deb
libquadmath0_11.1.0-4_amd64.deb
libstdc++-11-dev_11.1.0-4_amd64.deb
libstdc++6_11.1.0-4_amd64.deb
libtsan0_11.1.0-4_amd64.deb
libubsan1_11.1.0-4_amd64.deb

g++-11 https://packages.debian.org/experimental/g++-11

Install : cd Downloads/ ; # apt install ./*

That matters with your "Source.list".

if your "source.list" is the "Debian 11: Official sources.list":

deb http://deb.debian.org/debian/ bullseye main
deb-src http://deb.debian.org/debian/ bullseye main
deb http://security.debian.org/debian-security bullseye-security main contrib
deb-src http://security.debian.org/debian-security bullseye-security main contrib
deb http://deb.debian.org/debian/ bullseye-updates main contrib
deb-src http://deb.debian.org/debian/ bullseye-updates main contrib

then "apt-cache madison gcc":

gcc | 4:10.2.1-1 | http://deb.debian.org/debian bullseye/main amd64 Packages

gcc-defaults |      1.190 | http://deb.debian.org/debian bullseye/main Sources

If your "Source.list" is just like:

deb http://mirrors.xmission.com/debian/ testing main non-free contrib

deb http://http.us.debian.org/debian testing main contrib non-free

deb http://ftp.us.debian.org/debian testing main non-free contrib

then "apt-cache madison gcc":

gcc | 4:11.2.0-2 | http://mirrors.xmission.com/debian testing/main amd64 Packages

gcc | 4:11.2.0-2 | http://http.us.debian.org/debian testing/main amd64 Packages

gcc | 4:11.2.0-2 | http://ftp.us.debian.org/debian testing/main amd64 Packages

So, if you want to change your Debian 11's GCC from 10 to 11,

you could change that to your Debian "source.list".

Just do this below:

cd /etc/apt/ && sudo nano sources.list

And then change "source.list" with adding source.