This is a call for comments before I'd create a ticket in corresponding project (I'm not sure if it should be ghc or stack/cabal).
There's a common situation I get into: I compile with "stack build" (or cabal build), get a big list of warnings, fix few of them, then I re-compile project, but a lot of warnings are gone now. The only option I'm left with is to use "-fforce-recomp".