gcc-12 is not available in ubuntu 20.04, so we need to compile it from source code, here are the steps which I borrowed from this video:

• Step 1: clone gcc source code and checkout gcc-12 branch

$ git clone https://gcc.gnu.org/git/gcc.git gcc-source
$ cd gcc-source/
$ git branch -a
$ git checkout remotes/origin/releases/gcc-12

• Step 2: make another build dir

Note this is important as running ./configure from within the source directory is not supported as documented here.

$ mkdir ../gcc-12-build
$ cd ../gcc-12-build/
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 3: installing GCC prequisites and run configure again

The missing libraries will be shown in above ./confgiure output, search and install them one by one.

$ apt-cache search MPFR
$ sudo apt-get install libmpfrc++-dev
$ apt-cache search MPC | grep dev
$ sudo apt-get install libmpc-dev
$ apt-cache search GMP | grep dev
$ sudo apt-get install libgmp-dev
$ sudo apt-get install gcc-multilib
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

An alternative is to run the download_prerequisites script.

 cd gcc-source/
$ ./contrib/download_prerequisites
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++

• Step 4: compile gcc-12

$ make -j16

Still flex is missing:

$ sudo apt-get install flex
$ ./../gcc-source/configure --prefix=$HOME/install/gcc-12 --enable-languages=c,c++
$ make -j16
$ make install

Another way is to use Ubuntu 22.04 where gcc-12 is available. In Ubuntu 22.04, gcc-12 can be installed with apt:

$ sudo apt install gcc-12

If you were considering updating to a more recent long-term supported (LTS) version of Ubuntu, it might be a good time to do it now: Ubuntu 22.04LTS allows you to just install GCC 12's g++ using apt install g++-12, done. I predict that unless you have a lot of software depending on obsolete dependency libraries, updating your Ubuntu might be the easiest way to get g++12! (You can easily try that out in a podman or docker container, which I'll not go into here.)

If you're stuck to Ubuntu 20.04, I guess the cleanest and most straightforward way of getting g++12 will be to try and backport the 22.04 g++12 package. In that case, you don't have to worry about learning how to build GCC, and how to install a competing g++ that doesn't break all your system by interfering with existing compilers and even worse ABIs. Debian (and Ubuntu as kind-of-downstream of that) have taken the workload of learning that, and you just need to use their tools. Awesome!

Roughly speaking, the procedure to do that is this:

1. Prepare to build a Debian/Ubuntu package:

   sudo apt update
   sudo apt install -y build-essential fakeroot dpkg-dev devscripts cmake debhelper-compat dh-python equivs
   

2. Get the package sources. You will need to find the link to the .dsc file on the package page:

   mkdir gcc-12
   cd gcc-12
   dget -u https://URL/OF/DSC/FILE.dsc
   

3. Install the build dependencies from the .dsc:

   mk-build-deps -i gcc-12*.dsc
   

4. Build the packages:

   cd gcc-12-somethingsomething
   dpkg-buildpackage -us -uc -ui -d
   

So far for theory. In practice, step 3 will fail – GCC12 depends on tools whose version shipped by Ubuntu 20.04 is simply too old. Especially, the Debian package says you'd need at least g++11, binutils >= 2.37.

This is really the point where I personally say "If some software that I build needs a bleeding edge C++ compiler, it won't be happy with my Ubuntu 20.04 environment, anyway, and I need to update to a more modern Ubuntu". So, again that's my recommendation; update to Ubuntu 22.04, and then apt install g++-12.

If you want to live with the hassle, you will start to modify the gcc-12-12.*/debian/control and rules.conf, rules2 files to depend on just some g++ instead of g++11, disable ADA support, be less demanding of binutils etc.

However, you can be darn sure that the Debian folks thought about which versions of dependencies to use, and what fixes and backports to include. This is a rocky road. Probably less rocky than a "bare" source build, but it's an easy way to learn way more about GCC than you ever intended, and spend a couple of hours just to start the build.

Honestly, I keep repeating myself, but: if you suddenly find yourself in need of a modern compiler, it's a good time to update to a modern environment. That's way less pain and has way more benefits.

This question has been answered by @Marcus Müller in the Unix & Linux question How to install g++12 on Ubuntu 20.04? Essentially, Marcus Müller says that it may be possible to install g++12 on Ubuntu 20.04, but it's very hard to do, so it is strongly recommended that you use a later version of Ubuntu, such as 22.04, which supports g++12.

I'm quoting the whole answer below:

If you were considering updating to a more recent long-term supported (LTS) version of Ubuntu, it might be a good time to do it now: Ubuntu 22.04LTS allows you to just install GCC 12's g++ using apt install g++-12, done. I predict that unless you have a lot of software depending on obsolete dependency libraries, updating your Ubuntu might be the easiest way to get g++12! (You can easily try that out in a podman or docker container, which I'll not go into here.)

If you're stuck to Ubuntu 20.04, I guess the cleanest and most straightforward way of getting g++12 will be to try and backport the 22.04 g++12 package. In that case, you don't have to worry about learning how to build GCC, and how to install a competing g++ that doesn't break all your system by interfering with existing compilers and even worse ABIs. Debian (and Ubuntu as kind-of-downstream of that) have taken the workload of learning that, and you just need to use their tools. Awesome!

Roughly speaking, the procedure to do that is this:

1. Prepare to build a Debian/Ubuntu package:

   sudo apt update
   sudo apt install -y build-essential fakeroot dpkg-dev devscripts cmake debhelper-compat dh-python equivs
   

2. Get the package sources. You will need to find the link to the .dsc file on the package page:

   mkdir gcc-12
   cd gcc-12
   dget -u https://URL/OF/DSC/FILE.dsc
   

3. Install the build dependencies from the .dsc:

   mk-build-deps -i gcc-12*.dsc
   

4. Build the packages:

   cd gcc-12-somethingsomething
   dpkg-buildpackage -us -uc -ui -d
   

So far for theory. In practice, step 3 will fail – GCC12 depends on tools whose version shipped by Ubuntu 20.04 is simply too old. Especially, the Debian package says you'd need at least g++11, binutils >= 2.37.

This is really the point where I personally say "If some software that I build needs a bleeding edge C++ compiler, it won't be happy with my Ubuntu 20.04 environment, anyway, and I need to update to a more modern Ubuntu". So, again that's my recommendation; update to Ubuntu 22.04, and then apt install g++-12.

If you want to live with the hassle, you will start to modify the gcc-12-12.*/debian/control and rules.conf, rules2 files to depend on just some g++ instead of g++11, disable ADA support, be less demanding of binutils etc.

However, you can be darn sure that the Debian folks thought about which versions of dependencies to use, and what fixes and backports to include. This is a rocky road. Probably less rocky than a "bare" source build, but it's an easy way to learn way more about GCC than you ever intended, and spend a couple of hours just to start the build.

Honestly, I keep repeating myself, but: if you suddenly find yourself in need of a modern compiler, it's a good time to update to a modern environment. That's way less pain and has way more benefits.