A terrible hack which solves #3545.

Cabal/Distribution/Simple/PreProcess.hs

@@ -47,7 +47,7 @@ import Distribution.Verbosity
 
 import Control.Monad
 import Data.Maybe (fromMaybe)
-import Data.List (nub, isSuffixOf)
+import Data.List (nub, isSuffixOf, isPrefixOf)
 import System.Directory (doesFileExist)
 import System.Info (os, arch)
 import System.FilePath (splitExtension, dropExtensions, (</>), (<.>),
@@ -650,4 +650,21 @@ preprocessExtras comp lbi = case comp of
       BenchmarkUnsupported tt -> die $ "No support for preprocessing benchmark "
                                  ++ "type " ++ display tt
   where
-    pp dir = (map (dir </>) . concat) `fmap` forM knownExtrasHandlers ($ dir)
+    pp dir = (map (dir </>) . filter not_sub . concat) `fmap` forM knownExtrasHandlers ($ dir)
+    -- TODO: This is a terrible hack to work around #3545 while we don't
+    -- reorganize the directory layout.  Basically, for the main
+    -- library, we might accidentally pick up autogenerated sources for
+    -- our subcomponents, because they are all stored as subdirectories
+    -- in dist/build.  This is a cheap and cheerful check to prevent
+    -- this from happening.  It is not particularly correct; for example
+    -- if a user has a test suite named foobar and puts their C file in
+    -- foobar/foo.c, this test will incorrectly exclude it.  But I
+    -- didn't want to break BC...
+    not_sub p = and [ not (pre `isPrefixOf` p) | pre <- component_dirs ]
+    component_dirs = component_names (localPkgDescr lbi)
+    -- TODO: libify me
+    component_names pkg_descr =
+        map libName (libraries pkg_descr) ++
+        map exeName (executables pkg_descr) ++
+        map testName (testSuites pkg_descr) ++
+        map benchmarkName (benchmarks pkg_descr)