Sometimes we'll have a project that uses Go modules but one of its dependencies do not. Updating your project when its dependencies change can be awkward because there's no easy way of merging dependency requirements from non-module build systems such as dep.
The gomodmerge tool makes it possible to do that. In your non-module dependency, run go mod init to create a module file for that dependency. Then in your project, run gomodmerge $dependency/go.mod to merge any versions that are newer.