#15385: -Wincomplete-patterns gets confused when combining GADTs and pattern guards -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 Resolution: | Keywords: | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4968 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Gah. The deeper I dig the less happy I am. The `addDictsDs` is ok, but I'm worried about `genCaseTmCs1` and `genCaseTmCs2` The basic idea here is that when we see (in source Haskell) {{{ case x of { (a, Just b) -> blah } }}} we want to desugar `blah` in the knowledge that {{{ x ~ (a, Just b) }}} Reason: suppose that somewhere inside `blah` we see {{{ case x of { (a, Nothing) -> blah2 } }}} Then we want to declare this branch as unreachable. But what if the original expression was {{{ case x of (a, Just _) -> blah }}} Now what equality do we generate? Maybe we make a fresh variable? And maybe that is waht we want. This conversion from a pattern to a `PmExpr` is all hidden inside the cryptic line {{{ [e] <- map vaToPmExpr . coercePatVec <$> translatePat fam_insts p }}} I have very little idea of what this does. Returning to our case expression, we'll end up using `genCaseTmCs2` in `matchWrapper`. And that seems to generate ''two'' equalities {{{ v ~ (a, Just b) v ~ x }}} That seems terribly indirect, although perhaps not actually wrong. I suppose that it might work in cases like {{{ case (p,q) of (a, Just b) -> blah }}} but they probably never happen -- and at very least it could do with explanation. Bottom line so far: no actual bugs, but pretty tough going. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15385#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler