#16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes -------------------------------------+------------------------------------- Reporter: Ashley Yakeley | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Resolution: | Keywords: | QuantifiedConstraints, | ImpredicativeTypes Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by osa1): Sorry if I'm talking about a different bug, but I think this may be a regression. The original reproducer is rejected by GHC 8.6.3 too, but if you build Cabal (the library) with GHC HEAD at some point you get this error: {{{ $ head.hackage.sh init-local $ cabal new-build --enable-benchmarks --enable-tests --allow-newer=base ,template-haskell,tagged ... [211 of 220] Compiling Distribution.Simple.PreProcess ( Distribution/Simple/PreProcess.hs, dist/build/Distribution/Simple/PreProcess.o ) Distribution/Simple/PreProcess.hs:697:24: error: • Illegal qualified type: GHC.Stack.Types.HasCallStack => ghc-prim-0.5.3:GHC.Types.IO [FilePath] GHC doesn't yet support impredicative polymorphism • In the expansion of type synonym ‘WithCallStack’ In the expansion of type synonym ‘IO’ In the expansion of type synonym ‘PreProcessorExtras’ | 697 | knownExtrasHandlers :: [ PreProcessorExtras ] | ^^^^^^^^^^^^^^^^^^^^^^ cabal: Failed to build Cabal-2.4.1.0 (which is required by exe:ppsh from pretty-show-1.6.16 and cabal-install-2.5.0.0). See the build log above for details. }}} This builds fine with GHC 8.6.3. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16140#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler