Make check deal correctly with multiple branches

`cabal check` had a problem recognising fields in presence of
multiple branches. This patch fixes the problem and does not
meaningfully increases CI time of particularly taxing tests
(like “duplicate flagged dependencies” from MemoryUsage).

Cabal/src/Distribution/PackageDescription/Check/Conditional.hs

@@ -58,15 +58,16 @@ updateTargetAnnotation t ta = ta{taTarget = taTarget ta <> t}
 -- doc for more info).
 annotateCondTree
   :: forall a
-   . Monoid a
+   . (Eq a, Monoid a)
   => [PackageFlag] -- User flags.
   -> TargetAnnotation a
   -> CondTree ConfVar [Dependency] a
   -> CondTree ConfVar [Dependency] (TargetAnnotation a)
 annotateCondTree fs ta (CondNode a c bs) =
   let ta' = updateTargetAnnotation a ta
       bs' = map (annotateBranch ta') bs
-   in CondNode ta' c bs'
+      bs'' = crossAnnotateBranches defTrueFlags bs'
+   in CondNode ta' c bs''
   where
     annotateBranch
       :: TargetAnnotation a
@@ -107,12 +108,53 @@ annotateCondTree fs ta (CondNode a c bs) =
           )
           fs
 
+    defTrueFlags :: [PackageFlag]
+    defTrueFlags = filter flagDefault fs
+
+-- Propagate contextual information in CondTree branches. This is
+-- needed as CondTree is a rosetree and not a binary tree.
+crossAnnotateBranches
+  :: forall a
+   . (Eq a, Monoid a)
+  => [PackageFlag] -- `default: true` flags.
+  -> [CondBranch ConfVar [Dependency] (TargetAnnotation a)]
+  -> [CondBranch ConfVar [Dependency] (TargetAnnotation a)]
+crossAnnotateBranches fs bs = map crossAnnBranch bs
+  where
+    crossAnnBranch
+      :: CondBranch ConfVar [Dependency] (TargetAnnotation a)
+      -> CondBranch ConfVar [Dependency] (TargetAnnotation a)
+    crossAnnBranch wr =
+      let
+        rs = filter (/= wr) bs
+        ts = mapMaybe realiseBranch rs
+       in
+        utaBranch (mconcat ts) wr
+
+    realiseBranch :: CondBranch ConfVar [Dependency] (TargetAnnotation a) -> Maybe a
+    realiseBranch b =
+      let -- We are only interested in True by default package flags.
+          rbf :: ConfVar -> Either ConfVar Bool
+          rbf (PackageFlag n) | elem n (map flagName fs) = Right True
+          rbf _ = Right False
+          ms = simplifyCondBranch rbf (fmap taTarget b)
+       in fmap snd ms
+
+    utaBranch
+      :: a
+      -> CondBranch ConfVar [Dependency] (TargetAnnotation a)
+      -> CondBranch ConfVar [Dependency] (TargetAnnotation a)
+    utaBranch a (CondBranch k t mt) =
+      let utt (CondNode ka c wbs) =
+            (CondNode (updateTargetAnnotation a ka) c wbs)
+       in CondBranch k (utt t) (utt <$> mt)
+
 -- | A conditional target is a library, exe, benchmark etc., destructured
 -- in a CondTree. Traversing method: we render the branches, pass a
 -- relevant context, collect checks.
 checkCondTarget
   :: forall m a
-   . (Monad m, Monoid a)
+   . (Monad m, Eq a, Monoid a)
   => [PackageFlag] -- User flags.
   -> (a -> CheckM m ()) -- Check function (a = target).
   -> (UnqualComponentName -> a -> a)