Comments only

compiler/deSugar/DsBinds.hs

@@ -184,8 +184,9 @@ dsHsBind dflags (AbsBinds { abs_tvs = tyvars, abs_ev_vars = dicts
                           , abs_binds = binds, abs_sig = has_sig })
   = do { ds_binds <- addDictsDs (listToBag dicts) $
                      dsLHsBinds binds
-                                   -- addDictsDs: push type constraints deeper
-                                   --             for inner pattern match check
+                         -- addDictsDs: push type constraints deeper
+                         --             for inner pattern match check
+                         -- See Check, Note [Type and Term Equality Propagation]
 
        ; ds_ev_binds <- dsTcEvBinds_s ev_binds
 

compiler/typecheck/TcRnTypes.hs

@@ -385,9 +385,15 @@ instance ContainsModule DsGblEnv where
 data DsLclEnv = DsLclEnv {
         dsl_meta    :: DsMetaEnv,        -- Template Haskell bindings
         dsl_loc     :: RealSrcSpan,      -- To put in pattern-matching error msgs
-        dsl_dicts   :: Bag EvVar,        -- Constraints from GADT pattern-matching
-        dsl_tm_cs   :: Bag SimpleEq,
-        dsl_pm_iter :: IORef Int         -- no iterations for pmcheck
+
+        -- See Note [Note [Type and Term Equality Propagation] in Check.hs
+        -- These two fields are augmented as we walk inwards,
+        -- through each patttern match in turn
+        dsl_dicts   :: Bag EvVar,     -- Constraints from GADT pattern-matching
+        dsl_tm_cs   :: Bag SimpleEq,  -- Constraints form term-level pattern matching
+
+        dsl_pm_iter :: IORef Int  -- Number of iterations for pmcheck so far
+                                  -- We fail if this gets too big
      }
 
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