The "valid hole fits" machinery never suggests (non unidirectional) pattern synonyms. This can be easily seen from the code that simply never handles the `PatSynCon` case, see [here](https://gitlab.haskell.org/ghc/ghc/-/blob/6a43f8ec2e679d1240ddf6de597bc2ff55d134ca/compiler/GHC/Tc/Errors/Hole.hs#L808). This is straighforward to fix. The only slight subtletly is that we don't have an `Id` for a pattern synonym expression, so we have to work on the level of `Name`s instead. I am fixing in !14357.