Document typechecking of pattern bindings

compiler/typecheck/TcBinds.lhs

@@ -793,6 +793,28 @@ for a non-overloaded function.
 @tcMonoBinds@ deals with a perhaps-recursive group of HsBinds.
 The signatures have been dealt with already.
 
+Note [Pattern bindings]
+~~~~~~~~~~~~~~~~~~~~~~~
+The rule for typing pattern bindings is this:
+
+    ..sigs..
+    p = e
+
+where 'p' binds v1..vn, and 'e' may mention v1..vn, 
+typechecks exactly like
+
+    ..sigs..
+    x = e       -- Inferred type
+    v1 = case x of p -> v1
+    ..
+    vn = case x of p -> vn
+
+Note that  
+    (f :: forall a. a -> a) = id
+should not typecheck because
+       case id of { (f :: forall a. a->a) -> f }
+will not typecheck.
+
 \begin{code}
 tcMonoBinds :: TcSigFun -> LetBndrSpec 
             -> RecFlag  -- Whether the binding is recursive for typechecking purposes

compiler/typecheck/TcHsType.lhs

@@ -1113,8 +1113,7 @@ tcPatSig ctxt sig res_ty
 	-- that should be brought into scope
 
 	; if null sig_tvs then do {
-		-- The type signature binds no type variables, 
-		-- and hence is rigid, so use it to zap the res_ty
+		-- Just do the subsumption check and return
                   wrap <- tcSubType PatSigOrigin ctxt res_ty sig_ty
 		; return (sig_ty, [], wrap)
         } else do {