Ben Gamari pushed to branch wip/T26258 at Glasgow Haskell Compiler / GHC Commits: fbc34821 by Simon Peyton Jones at 2025-08-05T09:29:54-04:00 In TcSShortCut, typechecker plugins should get empty Givens Solving in TcShortCut mode means /ignoring the Givens/. So we should not pass them to typechecker plugins! Fixes #26258. This is a fixup to the earlier MR: commit 1bd12371feacc52394a0e660ef9349f9e8ee1c06 Author: Simon Peyton Jones Date: Mon Jul 21 10:04:49 2025 +0100 Improve treatment of SPECIALISE pragmas -- again! - - - - - 3 changed files: - compiler/GHC/Tc/Solver/Dict.hs - compiler/GHC/Tc/Solver/Monad.hs - compiler/GHC/Tc/Solver/Solve.hs Changes: ===================================== compiler/GHC/Tc/Solver/Dict.hs ===================================== @@ -496,11 +496,15 @@ We could use the Eq [a] superclass of the Ord [a], or we could use the top-level instance `Eq a => Eq [a]`. But if we did the latter we'd be stuck with an insoluble constraint (Eq a). -So the ShortCutSolving rule is this: +----------------------------------- +So the ShortCutSolving plan is this: If we could solve a constraint from a local Given, - try first to /completely/ solve the constraint using only top-level instances. + try first to /completely/ solve the constraint + using only top-level instances, + /without/ using any local Givens. - If that succeeds, use it - If not, use the local Given +----------------------------------- An example that succeeds: @@ -555,7 +559,7 @@ The moving parts are relatively simple: - `matchLocalInst`, which would otherwise consult Given quantified constraints - `GHC.Tc.Solver.Instance.Class.matchInstEnv`: when short-cut solving, don't pick overlappable top-level instances - + - `GHC.Tc.Solver.Solve.runTcPluginsWanted`: don't pass any Givens to the plugin Some wrinkles: ===================================== compiler/GHC/Tc/Solver/Monad.hs ===================================== @@ -897,14 +897,19 @@ for it, so TcS carries a mutable location where the binding can be added. This is initialised from the innermost implication constraint. -} --- | See Note [TcSMode] -data TcSMode +data TcSMode -- ^ See Note [TcSMode], where each constructor is documented = TcSVanilla -- ^ Normal constraint solving | TcSPMCheck -- ^ Used when doing patterm match overlap checks | TcSEarlyAbort -- ^ Abort early on insoluble constraints | TcSShortCut -- ^ Fully solve all constraints, without using local Givens deriving (Eq) +instance Outputable TcSMode where + ppr TcSVanilla = text "TcSVanilla" + ppr TcSPMCheck = text "TcSPMCheck" + ppr TcSEarlyAbort = text "TcSEarlyAbort" + ppr TcSShortCut = text "TcSShortcut" + {- Note [TcSMode] ~~~~~~~~~~~~~~~~~ The constraint solver can operate in different modes: ===================================== compiler/GHC/Tc/Solver/Solve.hs ===================================== @@ -1011,9 +1011,17 @@ solveSimpleGivens givens solveSimpleWanteds :: Cts -> TcS Cts -- The result is not necessarily zonked solveSimpleWanteds simples - = do { traceTcS "solveSimpleWanteds {" (ppr simples) + = do { mode <- getTcSMode ; dflags <- getDynFlags + ; inerts <- getInertSet + + ; traceTcS "solveSimpleWanteds {" $ + vcat [ text "Mode:" <+> ppr mode + , text "Inerts:" <+> ppr inerts + , text "Wanteds to solve:" <+> ppr simples ] + ; (n,wc) <- go 1 (solverIterations dflags) simples + ; traceTcS "solveSimpleWanteds end }" $ vcat [ text "iterations =" <+> ppr n , text "residual =" <+> ppr wc ] @@ -1663,19 +1671,28 @@ runTcPluginsGiven -- 'solveSimpleWanteds' should feed the updated wanteds back into the -- main solver. runTcPluginsWanted :: Cts -> TcS (Bool, Cts) -runTcPluginsWanted simples1 - | isEmptyBag simples1 - = return (False, simples1) +runTcPluginsWanted wanted + | isEmptyBag wanted + = return (False, wanted) | otherwise = do { solvers <- getTcPluginSolvers - ; if null solvers then return (False, simples1) else - - do { given <- getInertGivens - ; wanted <- TcS.zonkSimples simples1 -- Plugin requires zonked inputs - - ; traceTcS "Running plugins (" (vcat [ text "Given:" <+> ppr given - , text "Wanted:" <+> ppr wanted ]) - ; p <- runTcPluginSolvers solvers (given, bagToList wanted) + ; if null solvers then return (False, wanted) else + + do { -- Find the set of Givens to give to the plugin. + -- If TcSMode = TcSShortCut, we are solving with + -- no Givens so don't return any (#26258)! + -- See Note [Shortcut solving] in GHC.Tc.Solver.Dict + mode <- getTcSMode + ; given <- case mode of + TcSShortCut -> return [] + _ -> getInertGivens + + -- Plugin requires zonked input wanteds + ; zonked_wanted <- TcS.zonkSimples wanted + + ; traceTcS "Running plugins {" (vcat [ text "Given:" <+> ppr given + , text "Wanted:" <+> ppr zonked_wanted ]) + ; p <- runTcPluginSolvers solvers (given, bagToList zonked_wanted) ; let (_, solved_wanted) = pluginSolvedCts p (_, unsolved_wanted) = pluginInputCts p new_wanted = pluginNewCts p @@ -1684,9 +1701,6 @@ runTcPluginsWanted simples1 listToBag unsolved_wanted `andCts` listToBag insols --- SLPJ: I'm deeply suspicious of this --- ; updInertCans (removeInertCts $ solved_givens) - ; mapM_ setEv solved_wanted ; traceTcS "Finished plugins }" (ppr new_wanted) View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/fbc34821815b0b78d6b97ee441874258... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/fbc34821815b0b78d6b97ee441874258... 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