Fix #3294, work around buggy relinking by removing link target.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Cabal/Cabal.cabal

@@ -164,6 +164,7 @@ extra-source-files:
   tests/PackageTests/PreProcessExtraSources/Main.hs
   tests/PackageTests/PreProcessExtraSources/my.cabal
   tests/PackageTests/ReexportedModules/ReexportedModules.cabal
+  tests/PackageTests/Regression/T3294/T3294.cabal
   tests/PackageTests/TemplateHaskell/dynamic/Exe.hs
   tests/PackageTests/TemplateHaskell/dynamic/Lib.hs
   tests/PackageTests/TemplateHaskell/dynamic/TH.hs

Cabal/Distribution/Simple/GHC.hs

@@ -95,7 +95,7 @@ import Data.Monoid as Mon       ( Monoid(..) )
 import Data.Version             ( showVersion )
 import System.Directory
          ( doesFileExist, getAppUserDataDirectory, createDirectoryIfMissing
-         , canonicalizePath )
+         , canonicalizePath, removeFile )
 import System.FilePath          ( (</>), (<.>), takeExtension
                                 , takeDirectory, replaceExtension
                                 , isRelative )
@@ -958,7 +958,13 @@ buildOrReplExe forRepl verbosity numJobs _pkg_descr lbi
   -- link:
   unless forRepl $ do
     info verbosity "Linking..."
-    runGhcProg linkOpts { ghcOptOutputFile = toFlag (targetDir </> exeNameReal) }
+    -- Work around old GHCs not relinking in this
+    -- situation, see #3294
+    let target = targetDir </> exeNameReal
+    when (compilerVersion comp < Version [7,7] []) $ do
+      e <- doesFileExist target
+      when e (removeFile target)
+    runGhcProg linkOpts { ghcOptOutputFile = toFlag target }
 
 -- | Returns True if the modification date of the given source file is newer than
 -- the object file we last compiled for it, or if no object file exists yet.

Cabal/tests/PackageTests/Regression/T3294/.gitignore

+*.hs

Cabal/tests/PackageTests/Regression/T3294/T3294.cabal

+name:                T3294
+version:             0.1.0.0
+license:             BSD3
+author:              Edward Z. Yang
+maintainer:          ezyang@cs.stanford.edu
+build-type:          Simple
+cabal-version:       >=1.10
+
+executable T3294
+  main-is:             Main.hs
+  build-depends:       base
+  default-language:    Haskell2010

Cabal/tests/PackageTests/Tests.hs

@@ -344,6 +344,18 @@ tests config = do
       runExe' "hello-world" []
         >>= assertOutputContains "hello from A"
 
+  -- Test that executable recompilation works
+  -- https://github.com/haskell/cabal/issues/3294
+  tc "Regression/T3294" $ do
+    pkg_dir <- packageDir
+    liftIO $ writeFile (pkg_dir </> "Main.hs") "main = putStrLn \"aaa\""
+    cabal "configure" []
+    cabal "build" []
+    runExe' "T3294" [] >>= assertOutputContains "aaa"
+    liftIO $ writeFile (pkg_dir </> "Main.hs") "main = putStrLn \"bbb\""
+    cabal "build" []
+    runExe' "T3294" [] >>= assertOutputContains "bbb"
+
   where
     ghc_pkg_guess bin_name = do
         cwd <- packageDir