Make sure preprocessed module are on the module path

ghc-mod.cabal

@@ -78,6 +78,9 @@ Extra-Source-Files:     ChangeLog
                         test/data/custom-cradle/package-db-a/.gitkeep
                         test/data/custom-cradle/package-db-b/.gitkeep
                         test/data/custom-cradle/package-db-c/.gitkeep
+                        test/data/cabal-preprocessors/*.cabal
+                        test/data/cabal-preprocessors/*.hs
+                        test/data/cabal-preprocessors/*.hsc
 
 Library
   Default-Language:     Haskell2010

test/CheckSpec.hs

@@ -57,3 +57,10 @@ spec = do
             withDirectory_ "test/data/check-missing-warnings" $ do
                 res <- runD $ checkSyntax ["DesugarWarnings.hs"]
                 res `shouldBe` "DesugarWarnings.hs:4:9:Warning: Pattern match(es) are non-exhaustive\NULIn a case alternative: Patterns not matched: _ : _\n"
+
+        it "works with cabal builtin preprocessors" $ do
+            withDirectory_ "test/data/cabal-preprocessors" $ do
+                _ <- system "cabal clean"
+                _ <- system "cabal build"
+                res <- runD $ checkSyntax ["Main.hs"]
+                res `shouldBe` "Preprocessed.hsc:3:1:Warning: Top-level binding with no type signature: warning :: ()\n"

test/data/cabal-preprocessors/Main.hs

@@ -0,0 +1,4 @@
+import Preprocessed
+
+main :: IO ()
+main = return warning

test/data/cabal-preprocessors/Preprocessed.hsc

@@ -0,0 +1,3 @@
+module Preprocessed where
+
+warning = ()

test/data/cabal-preprocessors/cabal-preprocessors.cabal

@@ -0,0 +1,14 @@
+name:                cabal-preprocessors
+version:             0.1.0.0
+license-file:        LICENSE
+author:              asd
+maintainer:          asd
+build-type:          Simple
+cabal-version:       >=1.10
+
+executable cabal-preprocessors
+  main-is:             Main.hs
+  build-depends:       base
+  default-language:    Haskell2010
+  other-modules:       Preprocessed
+  ghc-options:         -Wall