Unable to install build-essential package on Ubuntu 20.04

Lately, I’ve been working mainly on a Windows 10 machine, and using WSL for all my Linux needs. I have Ubunt 20.04 installed under WSL, working with some Java and python apps, but today I wanted to install the build-essential package but was unable to:

$ sudo apt install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 build-essential : Depends: libc6-dev but it is not going to be installed or
                            libc-dev
                   Depends: g++ (>= 4:9.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Note: if you have this exact problem and are in a hurry, just go to the last paragraph to see the commands to execute to have build-essential installed.

Two dependencies could not be met: libc6-dev and g++. This is weird. I haven’t had this problem before (build-essential is a quite common package for Ubuntu installations), I’m not sure if this affects a “native” Ubuntu installation (or a non WSL one), but it probably does.

Tried to install libc6-dev:

$ sudo apt install libc6-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 libc6-dev : Depends: libc6 (= 2.31-0ubuntu9.2) but 2.31-0ubuntu9.3 is to be installed
E: Unable to correct problems, you have held broken packages.

Ok, libc6-dev depends on libc6-dev, but it seems the versions do not match: we have libc6 version 2.31-0ubuntu9.3 but libc6-dev depends on the previous 2.31-0ubuntu9.2.

We can install this previous libc6 version (downgrade it), first check if it’s still available. The -a option for the show command shows all available versions of a package:

$ apt show -a libc6
Package: libc6
Version: 2.31-0ubuntu9.3
Status: install ok installed
Priority: required
Section: libs
Source: glibc
Maintainer: Ubuntu Developers 
Original-Maintainer: GNU Libc Maintainers 
Installed-Size: 13.6 MB
Depends: libgcc-s1, libcrypt1 (>= 1:4.4.10-10ubuntu4)
Recommends: libidn2-0 (>= 2.0.5~)
Suggests: glibc-doc, debconf | debconf-2.0, locales
Conflicts: openrc (<< 0.27-2~)
Breaks: hurd (<< 1:0.9.git20170910-1), iraf-fitsutil (<< 2018.07.06-4), libtirpc1 (<< 0.2.3), locales (<< 2.31), locales-all (<< 2.31), nocache (<< 1.1-1~), nscd (<< 2.31), r-cran-later (<< 0.7.5+dfsg-2), wcc (<< 0.0.2+dfsg-3)
Replaces: libc6-amd64
Homepage: https://www.gnu.org/software/libc/libc.html
Original-Vcs-Browser: https://salsa.debian.org/glibc-team/glibc
Original-Vcs-Git: https://salsa.debian.org/glibc-team/glibc.git
Download-Size: unknown
APT-Manual-Installed: yes
APT-Sources: /var/lib/dpkg/status
Description: GNU C Library: Shared libraries
 Contains the standard libraries that are used by nearly all programs on
 the system. This package includes shared versions of the standard C library
 and the standard math library, as well as many others.

Package: libc6
Version: 2.31-0ubuntu9.2
Priority: required
Section: libs
Source: glibc
Origin: Ubuntu
Maintainer: Ubuntu Developers 
Original-Maintainer: GNU Libc Maintainers 
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 13.6 MB
Depends: libgcc-s1, libcrypt1 (>= 1:4.4.10-10ubuntu4)
Recommends: libidn2-0 (>= 2.0.5~)
Suggests: glibc-doc, debconf | debconf-2.0, locales
Conflicts: openrc (<< 0.27-2~)
Breaks: hurd (<< 1:0.9.git20170910-1), iraf-fitsutil (<< 2018.07.06-4), libtirpc1 (<< 0.2.3), locales (<< 2.31), locales-all (<< 2.31), nocache (<< 1.1-1~), nscd (<< 2.31), r-cran-later (<< 0.7.5+dfsg-2), wcc (<< 0.0.2+dfsg-3)
Replaces: libc6-amd64
Homepage: https://www.gnu.org/software/libc/libc.html
Task: minimal
Original-Vcs-Browser: https://salsa.debian.org/glibc-team/glibc
Original-Vcs-Git: https://salsa.debian.org/glibc-team/glibc.git
Download-Size: 2715 kB
APT-Sources: http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
Description: GNU C Library: Shared libraries
 Contains the standard libraries that are used by nearly all programs on
 the system. This package includes shared versions of the standard C library
 and the standard math library, as well as many others.

Package: libc6
Version: 2.31-0ubuntu9
Priority: required
Section: libs
Source: glibc
Origin: Ubuntu
Maintainer: Ubuntu Developers 
Original-Maintainer: GNU Libc Maintainers 
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 13.6 MB
Depends: libgcc-s1, libcrypt1 (>= 1:4.4.10-10ubuntu4)
Recommends: libidn2-0 (>= 2.0.5~)
Suggests: glibc-doc, debconf | debconf-2.0, locales
Conflicts: openrc (<< 0.27-2~)
Breaks: hurd (<< 1:0.9.git20170910-1), iraf-fitsutil (<< 2018.07.06-4), libtirpc1 (<< 0.2.3), locales (<< 2.31), locales-all (<< 2.31), nocache (<< 1.1-1~), nscd (<< 2.31), r-cran-later (<< 0.7.5+dfsg-2), wcc (<< 0.0.2+dfsg-3)
Replaces: libc6-amd64
Homepage: https://www.gnu.org/software/libc/libc.html
Task: minimal
Download-Size: 2713 kB
APT-Sources: http://archive.ubuntu.com/ubuntu focal/main amd64 Packages
Description: GNU C Library: Shared libraries
 Contains the standard libraries that are used by nearly all programs on
 the system. This package includes shared versions of the standard C library
 and the standard math library, as well as many others.

We have versions 2.31-0ubuntu9.3, 2.31-0ubuntu9.2 and 2.31-0ubuntu9. Let's install the one required by libc6-dev:

$ sudo apt install libc6=2.31-0ubuntu9.2
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  glibc-doc
The following packages will be DOWNGRADED:
  libc6
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
Need to get 2715 kB of archives.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n]
Get:1 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 libc6 amd64 2.31-0ubuntu9.2 [2715 kB]
Fetched 2715 kB in 2s (1695 kB/s)
Preconfiguring packages ...
dpkg: warning: downgrading libc6:amd64 from 2.31-0ubuntu9.3 to 2.31-0ubuntu9.2
(Reading database ... 89025 files and directories currently installed.)
Preparing to unpack .../libc6_2.31-0ubuntu9.2_amd64.deb ...
Unpacking libc6:amd64 (2.31-0ubuntu9.2) over (2.31-0ubuntu9.3) ...
Setting up libc6:amd64 (2.31-0ubuntu9.2) ...
Processing triggers for libc-bin (2.31-0ubuntu9.3) ...

Let's try to install build-essential again:

$ sudo apt install build-essential
Reading package lists... Done
Building dependency tree
Reading state information... Done
Preparing to unpack .../12-libalgorithm-diff-xs-perl_0.04-6_amd64.deb ...
Unpacking libalgorithm-diff-xs-perl (0.04-6) ...
Selecting previously unselected package libalgorithm-merge-perl.
Preparing to unpack .../13-libalgorithm-merge-perl_0.08-3_all.deb ...
Unpacking libalgorithm-merge-perl (0.08-3) ...
Selecting previously unselected package manpages-dev.
Preparing to unpack .../14-manpages-dev_5.05-1_all.deb ...
Unpacking manpages-dev (5.05-1) ...
Setting up manpages-dev (5.05-1) ...
Setting up libalgorithm-diff-perl (1.19.03-2) ...
Setting up linux-libc-dev:amd64 (5.4.0-86.97) ...
Setting up libfakeroot:amd64 (1.24-1) ...
Setting up dpkg-dev (1.19.7ubuntu3) ...
Setting up fakeroot (1.24-1) ...
update-alternatives: using /usr/bin/fakeroot-sysv to provide /usr/bin/fakeroot (fakeroot) in auto mode
Setting up libcrypt-dev:amd64 (1:4.4.10-10ubuntu4) ...
Setting up libc-dev-bin (2.31-0ubuntu9.2) ...
Setting up libalgorithm-diff-xs-perl (0.04-6) ...
Setting up libalgorithm-merge-perl (0.08-3) ...
Setting up libc6-dev:amd64 (2.31-0ubuntu9.2) ...
Setting up libstdc++-9-dev:amd64 (9.3.0-17ubuntu1~20.04) ...
Setting up g++-9 (9.3.0-17ubuntu1~20.04) ...
Setting up g++ (4:9.3.0-1ubuntu2) ...
update-alternatives: using /usr/bin/g++ to provide /usr/bin/c++ (c++) in auto mode
Setting up build-essential (12.8ubuntu1.1) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for libc-bin (2.31-0ubuntu9.3) ...

Done!

In summary: you need to downgrade libc6 package to version 2.31-0ubuntu9.2, then you'll be able to install build-essential and its dependencies.

$ sudo apt install libc6=2.31-0ubuntu9.2
$ sudo apt install build-essential