Add -Wcpp-undef warning flagwip/erikd/cpp-undef

Summary:
When enabled, this new warning flag passes `-Wundef` to the C
pre-processor which causes the pre-processor to warn on uses of
the `#if` directive on undefined identifiers.

It is not currently enabled in any of the standard warning groups.

Test Plan:
Add a test to this commit and make sure the test passes
on all major platforms.

Reviewers: bgamari, hvr, austin, Phyx, carter

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2626

GHC Trac Issues: #12752

7 files changed, 34 insertions, 4 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 7978c03352..cd8dc41ceb 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -619,6 +619,7 @@ data WarningFlag =
    | Opt_WarnMissingPatternSynonymSignatures -- since 8.0
    | Opt_WarnUnrecognisedWarningFlags     -- since 8.0
    | Opt_WarnSimplifiableClassConstraints -- Since 8.2
+   | Opt_WarnCPPUndef                     -- Since 8.2
    deriving (Eq, Show, Enum)
 
 data Language = Haskell98 | Haskell2010
@@ -3287,6 +3288,7 @@ wWarningFlagsDeps = [
     "it has no effect",
   depFlagSpec "auto-orphans"             Opt_WarnAutoOrphans
     "it has no effect",
+  flagSpec "cpp-undef"                   Opt_WarnCPPUndef,
   flagSpec "deferred-type-errors"        Opt_WarnDeferredTypeErrors,
   flagSpec "deferred-out-of-scope-variables"
                                          Opt_WarnDeferredOutOfScopeVariables,
diff --git a/compiler/main/SysTools.hs b/compiler/main/SysTools.hs
index dd98883cb5..d5fd0c5c29 100644
--- a/compiler/main/SysTools.hs
+++ b/compiler/main/SysTools.hs
@@ -403,9 +403,8 @@ runCpp :: DynFlags -> [Option] -> IO ()
 runCpp dflags args =   do
   let (p,args0) = pgm_P dflags
       args1 = map Option (getOpts dflags opt_P)
-      args2 = if gopt Opt_WarnIsError dflags
-                 then [Option "-Werror"]
-                 else []
+      args2 = [Option "-Werror" | gopt Opt_WarnIsError dflags]
+                ++ [Option "-Wundef" | wopt Opt_WarnCPPUndef dflags]
   mb_env <- getGccEnv args2
   runSomethingFiltered dflags id  "C pre-processor" p
                        (args0 ++ args1 ++ args2 ++ args) mb_env
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index c07058a4ef..c9216b9307 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -1007,6 +1007,12 @@ of ``-W(no-)*``.
     be inlined before the rule has a chance to fire. See
     :ref:`rules-inline`.
 
+.. ghc-flag:: -Wcpp-undef
+
+    This flag passes ``-Wundef`` to the C pre-processor (if its being used)
+    which causes the pre-processor to warn on uses of the `#if` directive on
+    undefined identifiers.
+
 If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice.
 It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
 sanity, not yours.)
diff --git a/testsuite/tests/driver/T12752pass.hs b/testsuite/tests/driver/T12752pass.hs
new file mode 100644
index 0000000000..5826d48b88
--- /dev/null
+++ b/testsuite/tests/driver/T12752pass.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE CPP #-}
+
+#if SHOULD_PASS
+message :: String
+message = "Hello!"
+#endif
+
+main :: IO ()
+main = putStrLn message
diff --git a/testsuite/tests/driver/all.T b/testsuite/tests/driver/all.T
index c6283df156..8cd5c2f968 100644
--- a/testsuite/tests/driver/all.T
+++ b/testsuite/tests/driver/all.T
@@ -491,9 +491,11 @@ test('T12135',
      run_command,
      ['$MAKE -s --no-print-directory T12135'])
 
-test('T12192', normal, run_command, ['mkdir foo && (cd foo && {compiler} -v0 ../T12192)']) 
+test('T12192', normal, run_command, ['mkdir foo && (cd foo && {compiler} -v0 ../T12192)'])
 
 test('T10923',
      extra_clean(['T10923.o', 'T10923.hi']),
      run_command,
      ['$MAKE -s --no-print-directory T10923'])
+
+test('T12752pass', normal, compile, ['-DSHOULD_PASS=1 -Wcpp-undef'])
diff --git a/testsuite/tests/driver/should_fail/T12752.hs b/testsuite/tests/driver/should_fail/T12752.hs
new file mode 100644
index 0000000000..3560d00894
--- /dev/null
+++ b/testsuite/tests/driver/should_fail/T12752.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE CPP #-}
+
+-- This should fail to compile with "ghc -Wcpp-undef -Werror ...".
+#if this_cpp_identifier_does_not_exist
+message :: String
+message = "This is wrong!"
+#endif
+
+main :: IO ()
+main = putStrLn "Hello"
diff --git a/testsuite/tests/driver/should_fail/all.T b/testsuite/tests/driver/should_fail/all.T
index f068d6516d..3d0708b285 100644
--- a/testsuite/tests/driver/should_fail/all.T
+++ b/testsuite/tests/driver/should_fail/all.T
@@ -1,2 +1,4 @@
 # --make -o without Main should be an error, not a warning.
 test('T10895', normal, multimod_compile_fail, ['T10895.hs', '-v0 -o dummy'])
+
+test('T12752', expect_fail, compile, ['-Wcpp-undef -Werror'])