diff --git a/compiler/typecheck/Inst.hs b/compiler/typecheck/Inst.hs
index 18a0e2fd47b8c95202e74eaa1eab7e450047b215..f4caf2b74a586d4c442fffa6959a5248fddc31ee 100644
--- a/compiler/typecheck/Inst.hs
+++ b/compiler/typecheck/Inst.hs
@@ -407,6 +407,9 @@ syntaxNameCtxt name orig ty tidy_env
 -}
 
 getOverlapFlag :: Maybe OverlapMode -> TcM OverlapFlag
+-- Construct the OverlapFlag from the global module flags,
+-- but if the overlap_mode argument is (Just m),
+--     set the OverlapMode to 'm'
 getOverlapFlag overlap_mode
   = do  { dflags <- getDynFlags
         ; let overlap_ok    = xopt Opt_OverlappingInstances dflags