Re-read pragmas after preprocessing (#2464, #3674, #3457)

This was a lot easier than I imagined.

compiler/main/DriverPipeline.hs

@@ -696,19 +696,30 @@ runPhase (Unlit sf) _stop hsc_env _basename _suff input_fn get_output_fn maybe_l
 runPhase (Cpp sf) _stop hsc_env _basename _suff input_fn get_output_fn maybe_loc
   = do let dflags0 = hsc_dflags hsc_env
        src_opts <- liftIO $ getOptionsFromFile dflags0 input_fn
-       (dflags, unhandled_flags, warns)
+       (dflags1, unhandled_flags, warns)
            <- liftIO $ parseDynamicNoPackageFlags dflags0 src_opts
-       handleFlagWarnings dflags warns
        checkProcessArgsResult unhandled_flags
 
-       if not (dopt Opt_Cpp dflags) then
+       if not (dopt Opt_Cpp dflags1) then do
+           -- we have to be careful to emit warnings only once.
+           unless (dopt Opt_Pp dflags1) $ handleFlagWarnings dflags1 warns
+
            -- no need to preprocess CPP, just pass input file along
 	   -- to the next phase of the pipeline.
-          return (HsPp sf, dflags, maybe_loc, input_fn)
+           return (HsPp sf, dflags1, maybe_loc, input_fn)
 	else do
-	    output_fn <- liftIO $ get_output_fn dflags (HsPp sf) maybe_loc
-	    liftIO $ doCpp dflags True{-raw-} False{-no CC opts-} input_fn output_fn
-	    return (HsPp sf, dflags, maybe_loc, output_fn)
+	    output_fn <- liftIO $ get_output_fn dflags1 (HsPp sf) maybe_loc
+	    liftIO $ doCpp dflags1 True{-raw-} False{-no CC opts-} input_fn output_fn
+            -- re-read the pragmas now that we've preprocessed the file
+            -- See #2464,#3457
+            src_opts <- liftIO $ getOptionsFromFile dflags0 output_fn
+            (dflags2, unhandled_flags, warns)
+                <- liftIO $ parseDynamicNoPackageFlags dflags0 src_opts
+            unless (dopt Opt_Pp dflags2) $ handleFlagWarnings dflags2 warns
+            -- the HsPp pass below will emit warnings
+            checkProcessArgsResult unhandled_flags
+
+	    return (HsPp sf, dflags2, maybe_loc, output_fn)
 
 -------------------------------------------------------------------------------
 -- HsPp phase 
@@ -730,7 +741,15 @@ runPhase (HsPp sf) _stop hsc_env basename suff input_fn get_output_fn maybe_loc
 			     ] ++
 			     map SysTools.Option hspp_opts
 			   )
-	    return (Hsc sf, dflags, maybe_loc, output_fn)
+
+            -- re-read pragmas now that we've parsed the file (see #3674)
+            src_opts <- liftIO $ getOptionsFromFile dflags output_fn
+            (dflags1, unhandled_flags, warns)
+                <- liftIO $ parseDynamicNoPackageFlags dflags src_opts
+            handleFlagWarnings dflags1 warns
+            checkProcessArgsResult unhandled_flags
+
+	    return (Hsc sf, dflags1, maybe_loc, output_fn)
 
 -----------------------------------------------------------------------------
 -- Hsc phase