Discussion on #ghc concluded:
- Haskell2010, Haskell98 extensions are mutually incompatible, i.e. it is an error if both appear in LANGUAGE pragmas in a source file. Incompatible values on the commandline are OK.
- Haskell2010, Haskell98 are applied before other extensions in LANGUAGE pragmas, on the commandline, etc
- GHC will default to Haskell2010