Fix haskell#7698: cabal sdist works when extra-packages are present

cabal-install/src/Distribution/Client/CmdSdist.hs

@@ -273,9 +273,16 @@ reifyTargetSelectors pkgs sels =
         ([], sels') -> Right sels'
         (errs, _)   -> Left errs
     where
-        flatten (SpecificSourcePackage pkg@SourcePackage{}) = pkg
-        flatten _ = error "The impossible happened: how do we not know about a local package?"
-        pkgs' = fmap flatten pkgs
+        -- there can be pkgs which are in extra-packages:
+        -- these are not SpecificSourcePackage
+        --
+        -- Why these packages are in localPkgs, it's confusing.
+        -- Anyhow, better to be lenient here.
+        --
+        flatten (SpecificSourcePackage pkg@SourcePackage{}) = Just pkg
+        flatten _                                           = Nothing
+
+        pkgs' = mapMaybe flatten pkgs
 
         getPkg pid = case find ((== pid) . packageId) pkgs' of
             Just pkg -> Right pkg

cabal-testsuite/PackageTests/SDist/T7698/cabal.out

@@ -0,0 +1,2 @@
+# cabal v2-sdist
+Wrote source list to <TMPDIR>/t7698-0.list

cabal-testsuite/PackageTests/SDist/T7698/cabal.project

@@ -0,0 +1,2 @@
+packages: pkg/*.cabal
+extra-packages: lens

cabal-testsuite/PackageTests/SDist/T7698/cabal.test.hs

@@ -0,0 +1,4 @@
+import Test.Cabal.Prelude
+main = cabalTest $ do
+  tmpdir <- fmap testTmpDir getTestEnv
+  cabal "v2-sdist" ["--ignore-project", "--list-only", "--output-directory", tmpdir, "all"]

cabal-testsuite/PackageTests/SDist/T7698/pkg/Main.hs

@@ -0,0 +1 @@
+main = putStrLn "hi"

cabal-testsuite/PackageTests/SDist/T7698/pkg/t7698.cabal

@@ -0,0 +1,7 @@
+cabal-version: 2.2
+name: t7698
+version: 0
+
+executable foo
+  default-language: Haskell2010
+  main-is: Main.hs

changelog.d/issue-7698

@@ -0,0 +1,3 @@
+synopsis: `cabal sdist` works in projects with `extra-packages`
+issues: #7698
+packages: cabal-install