Description:
makepkg can be configured to produce -debug packages containing only the debug symbols. These packages depend on their regular packages, e.g. foo-debug-1.0 depends on foo-debug=1.0, but they shouldn't.

There is no actual dependency:
The debug packages don't require their regular packages to function. They only add improved debugging functionality to the regular packages. It is more appropriate to have foo-1.0 optionally depend on foo-debug=1.0, but since debug packages are not provided in the repositories, the optdepend isn't very useful either.

Having the dependency causes pain for users:
When upgrading a package foo from 1.0 to 1.1 with foo-debug-1.0 installed, pacman will complain about unresolved dependency, and the user has to manually remove foo-debug first.

Reasons for having the dependency are not good enough:
After upgrading foo from 1.0 to 1.1, foo-debug-1.0 probably becomes useless, so maybe the unresolved conflict error serves as a way to remind the user to remove it. But this is not consistent with the way similar situations are handled. If foo-1.0 depends on libbar, and foo-1.1 does not depend on libbar, libbar becomes useless. But this is usually handled by installing libbar --asdeps, and not by having libbar depend on foo=1.0.

After upgrading foo from 1.0 to 1.1, the symbols in foo-debug-1.0 may become incorrect, and it can be argued that having the wrong debug package installed is worse than having no debug package installed. I haven't really tried this so I don't know how much worse it is, or if gdb is able to figure out the symbols file is wrong and not use it. But I think this is similar to -docs packages: linux-docs is specific to the same version of linux, but it doesn't depend on linux, and similar for other -docs packages. Usually a system upgrade will upgrade linux-docs at the same time as linux, but if the user specifically upgrades only linux and linux-docs, the docs will be out-of-date, but this doesn't really affect the functionality of the packages. If the user plans to debug foo-1.1, they should also install foo-debug-1.1, and if not, then having foo-debug-1.0 installed is harmless.