How can I identify the packages with conflicting dependencies?

Question

When performing sudo apt upgrade I get the following error message:

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:
 libcurl4 : Conflicts: libcurl3 but 7.60.0-1 is to be installed
E: Broken packages

I understand this to mean:

• You have previously installed package X, which relies on libcurl4 (so libcurl4 is already installed).
• Your upgrade includes a package Y with a new dependency on libcurl3 (version 7.60.0-1).
• libcurl3 cannot be installed because it conflicts with libcurl4.

I used GNOME Packages to look at the lists of packages that require libcurl3 and libcurl4, but the lists are very long and can't be narrowed down (specifically, they don't indicate which packages are already installed or which are about to be installed).

I tried apt debugging but it doesn't seem to add any information:

$ sudo apt -o Debug::pkgProblemResolver=yes upgrade                                                                                                           
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Entering ResolveByKeep 10%
  Dependencies are not satisfied for libcurl4:amd64 < 7.60.0-2 @ii mK Ib >
Keeping package libcurl4:amd64
  Dependencies are not satisfied for libcurl4:amd64 < 7.60.0-2 @ii mK Ib >
Package libcurl4:amd64 libcurl4:amd64 Conflicts on libcurl3:amd64 < none -> 7.60.0-1 @un uN >
  Dependencies are not satisfied for libcurl4:amd64 < 7.60.0-2 @ii mK Ib >
  Dependencies are not satisfied for libcurl4:amd64 < 7.60.0-2 @ii mK Ib >
  Dependencies are not satisfied for libcurl4:amd64 < 7.60.0-2 @ii mK Ib >
Calculating upgrade... Done
<repeats previous error message>

How can I identify package X (the existing package) and package Y (the new package, or an existing package with a new dependency)?

Answer 1

To identify packages causing conflicts (or other issues) during a package upgrade, I find the most helpful approach is to enable the resolver’s debugging mode:

apt -o Debug::pkgProblemResolver=yes upgrade

This will show you all the problems the resolver runs into, along with the packages which caused it to consider the problematic dependencies.

Answer 2

Another take to add to @StephenKitt excelent answer is installing and using aptitude.

aptitude is usually a bit smarter managing packages and specially helping and suggesting alternatives when there are conflicts.

The usage is also very similar to apt as in using, aptitude install, aptitude update and aptitude upgrade. If you call aptitude without arguments, it will invoke a text menu interface.

From man aptitude

aptitude - high-level interface to the package manager

aptitude is a text-based interface to the Debian GNU/Linux package system.

It allows the user to view the list of packages and to perform package management tasks such as installing, upgrading, and removing packages. Actions may be performed from a visual interface or from the command-line.

For more details see aptitude - Command-line reference