Driver: --make -o without Main should be an error (#10895)

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1253

compiler/main/GhcMake.hs

@@ -325,17 +325,19 @@ load how_much = do
             a_root_is_Main = any ((==main_mod).ms_mod) mod_graph
             do_linking = a_root_is_Main || no_hs_main || ghcLink dflags == LinkDynLib || ghcLink dflags == LinkStaticLib
 
-          when (ghcLink dflags == LinkBinary
-                && isJust ofile && not do_linking) $
-            liftIO $ debugTraceMsg dflags 1 $
-                text ("Warning: output was redirected with -o, " ++
-                      "but no output will be generated\n" ++
-                      "because there is no " ++
-                      moduleNameString (moduleName main_mod) ++ " module.")
-
           -- link everything together
           linkresult <- liftIO $ link (ghcLink dflags) dflags do_linking (hsc_HPT hsc_env1)
 
+          if ghcLink dflags == LinkBinary && isJust ofile && not do_linking
+             then do
+                liftIO $ errorMsg dflags $ text
+                   ("output was redirected with -o, " ++
+                    "but no output will be generated\n" ++
+                    "because there is no " ++
+                    moduleNameString (moduleName main_mod) ++ " module.")
+                -- This should be an error, not a warning (#10895).
+                loadFinish Failed linkresult
+             else
                 loadFinish Succeeded linkresult
 
      else

testsuite/tests/driver/should_fail/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk

testsuite/tests/driver/should_fail/T10895.hs

+module NotMain where

testsuite/tests/driver/should_fail/T10895.stderr

+
+<no location info>: error:
+    output was redirected with -o, but no output will be generated
+because there is no Main module.

testsuite/tests/driver/should_fail/all.T

+# --make -o without Main should be an error, not a warning.
+test('T10895', normal, multimod_compile_fail, ['T10895.hs', '-v0 -o dummy'])