Doing

ghc -M Foo.hs

gives

You must specify at least one -dep-suffix

I went to the documentation and noticed (7.8.12) an example:

depend :
        ghc -dep-suffix '' -M $(HC_OPTS) $(SRCS)

So, empty suffix is fine. Why not default to empty string then?

I found a commit which introduced the requirement (af072fc): it says, it fixed #7381 (closed), and this doesn't seem to do anything with dep-suffix in particular. So, maybe there is no good reason to require that.