#15116: GHC internal error when GADT return type mentions its own constructor name -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.5 checker) | Keywords: GADTs, Resolution: | TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple crash or panic | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Ah, I see what is going on. This is due to the following change in `kcLTyClDecl`: {{{#!diff kcLTyClDecl :: LTyClDecl GhcRn -> TcM () -- See Note [Kind checking for type and class decls] kcLTyClDecl (L loc decl) + | hsDeclHasCusk decl + = traceTc "kcTyClDecl skipped due to cusk" (ppr tc_name) + | otherwise = setSrcSpan loc $ tcAddDeclCtxt decl $ - do { traceTc "kcTyClDecl {" (ppr (tyClDeclLName decl)) + do { traceTc "kcTyClDecl {" (ppr tc_name) ; kcTyClDecl decl - ; traceTc "kcTyClDecl done }" (ppr (tyClDeclLName decl)) } + ; traceTc "kcTyClDecl done }" (ppr tc_name) } + where + tc_name = tyClDeclLName decl }}} Now, if the declaration has a CUSK, then `kcTyClDecl` is skipped entirely. As a consequence, subsequent checks for each data constructor (`kcConDecl`) are also skipped. But `kcConDecl` was what was throwing the error message for the original program, so skipping it leads to pandemonium later down the line. How should we fix this? I can think of at least a couple of possibilities: 1. Never skip `kcTyClDecl` if the declaration has constructors. 2. Do the `kcConDecl` checks separately from `kcTyClDecl`. Thoughts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15116#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler