#13768: Incorrect warnings generated by exhaustiveness checker with pattern synonyms / GADT combination -------------------------------------+------------------------------------- Reporter: kosmikus | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by mpickering): * priority: normal => high * keywords: => PatternSynonyms Comment: I think the problem here is in `mkOneConFull` which makes a substitution by zipping together the `univ_tvs` and `tc_args`. If I print out what these values are in this example I find that.. {{{ univ_tvs [k_a133, xs'_aSi, f_aSj] tc_args [k_a27Q, f_a27S, ys_a27R] }}} which then generates equalities like.. {{{ theta_cs [(f_a27S :: [k_a27Q]) ~ ((x_d2kG : xs_d2kH) :: [k_a27Q])] }}} and the constraint solver reports as unsatisfiable. If I insert a hack to swap these type variables into the "right" order then the program compiles without warning as expected. {{{ univ_tvs [k_a133, xs'_aSi, f_aSj] tc_args [k_a27Q, f_a27S, ys_a27R] theta_cs [(ys_a27R :: [k_a27Q]) ~ ((x_d2ky : xs_d2kz) :: [k_a27Q])] }}} I don't know what the right solution is here as in general there seems no reason for these two lists to appear in the right order. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13768#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler