Fix #5659: port instance Monad (TCMT m) to transformers-0.6

Agda.cabal

@@ -240,11 +240,9 @@ library
                 , haskeline >= 0.7.2.3 && < 0.9
                 -- monad-control-1.0.1.0 is the first to contain liftThrough
                 , monad-control >= 1.0.1.0 && < 1.1
-                -- mtl-2.1 contains a severe bug.
-                --
                 -- mtl >= 2.2 && < 2.2.1 doesn't export Control.Monad.Except.
                 -- Need mtl 2.2.2  for export of Control.Monad.IdentityT (ghc 8.2.2+)
-                , mtl >= 2.2.1 && < 2.3
+                , mtl >= 2.2.1 && < 2.4
                 , murmur-hash >= 0.1 && < 0.2
                 , parallel >= 3.2.2.0 && < 3.3
                 , pretty >= 1.1.3.3 && < 1.2
@@ -261,6 +259,12 @@ library
                 , uri-encode >= 1.5.0.4 && < 1.6
                 , zlib == 0.6.*
 
+  -- Andreas, 2022-02-02, issue #5659:
+  -- Build failure with transformers-0.6.0.{0,1,2} and GHC 8.6.
+  -- Transformers-0.6.0.3 might restored GHC 8.6 buildability.
+  if impl(ghc == 8.6.*)
+    build-depends: transformers < 0.6 || >= 0.6.0.3
+
   -- Andreas, 2021-03-10:
   -- All packages we depend upon should be mentioned in an unconditional
   -- build-depends field, but additional restrictions on their

src/full/Agda/TypeChecking/Monad/Base.hs

@@ -4244,7 +4244,14 @@ instance MonadTrans TCMT where
     lift m = TCM $ \_ _ -> m
 
 -- We want a special monad implementation of fail.
+#if __GLASGOW_HASKELL__ < 808
 instance MonadIO m => Monad (TCMT m) where
+#else
+-- Andreas, 2022-02-02, issue #5659:
+-- @transformers-0.6@ requires exactly a @Monad@ superclass constraint here
+-- if we want @instance MonadTrans TCMT@.
+instance Monad m => Monad (TCMT m) where
+#endif
     return = pure
     (>>=)  = bindTCMT
     (>>)   = (*>)

src/full/Agda/TypeChecking/Monad/Base.hs-boot

@@ -1,3 +1,5 @@
+{-# LANGUAGE CPP #-}
+
 module Agda.TypeChecking.Monad.Base where
 
 import Control.Monad.IO.Class (MonadIO)
@@ -28,9 +30,17 @@ newtype TCMT m a = TCM { unTCM :: IORef TCState -> TCEnv -> m a }
 
 instance Applicative m => Applicative (TCMT m)
 instance Functor m => Functor (TCMT m)
-instance MonadIO m => Monad (TCMT m)
 instance MonadIO m => MonadIO (TCMT m)
 
+#if __GLASGOW_HASKELL__ < 808
+instance MonadIO m => Monad (TCMT m) where
+#else
+-- Andreas, 2022-02-02, issue #5659:
+-- @transformers-0.6@ requires exactly a @Monad@ superclass constraint here
+-- if we want @instance MonadTrans TCMT@.
+instance Monad m => Monad (TCMT m) where
+#endif
+
 type TCM = TCMT IO
 
 type ModuleToSource = Map TopLevelModuleName AbsolutePath