compiler/main: fixes #9776

Test Plan: test T9776 under tests/driver

Reviewers: jstolarek, austin

Reviewed By: jstolarek, austin

Subscribers: jstolarek, thomie, carter

Differential Revision: https://phabricator.haskell.org/D503

GHC Trac Issues: #9776

compiler/main/CmdLineParser.hs

@@ -191,8 +191,9 @@ processOneArg opt_kind rest arg args
                                     []               -> missingArgErr dash_arg
                                     (L _ arg1:args1) -> Right (f arg1, args1)
 
+        -- See Trac #9776
         SepArg f -> case args of
-                        []               -> unknownFlagErr dash_arg
+                        []               -> missingArgErr dash_arg
                         (L _ arg1:args1) -> Right (f arg1, args1)
 
         Prefix f | notNull rest_no_eq -> Right (f rest_no_eq, args)

testsuite/tests/driver/T9776.hs

+module T9776 where

testsuite/tests/driver/T9776.stderr

+ghc-stage2: on the commandline: missing argument for flag: -frule-check
+Usage: For basic information, try the `--help' option.

testsuite/tests/driver/all.T

@@ -406,3 +406,6 @@ test('T9050', normal, build_T9050, [])
 
 test('write_interface_oneshot', normal, run_command, ['$MAKE -s --no-print-directory write_interface_oneshot'])
 test('write_interface_make', normal, run_command, ['$MAKE -s --no-print-directory write_interface_make'])
+
+test('T9776', normal, compile_fail, ['-frule-check'])
+