Do not lump together forall and constructors when deciding on layout

CHANGELOG.md

@@ -20,8 +20,9 @@
   expressions. [Issue 513](https://github.com/tweag/ormolu/issues/513).
 
 * Made rendering of data type definitions, value-level applications, and
-  application of types use the same style. [Issue
-  427](https://github.com/tweag/ormolu/issues/427).
+  application of types use the same style. Moreover, existential now doesn't
+  cause the data constructor be unconditionally rendered in multiline layout
+  [Issue 427](https://github.com/tweag/ormolu/issues/427).
 
 * Implemented support for the new language extension `ImportQualifiedPost`.
 

data/examples/declaration/data/existential-multiline-out.hs

@@ -3,3 +3,15 @@
 data Foo
   = forall a. MkFoo a (a -> Bool)
   | forall a. Eq a => MkBar a
+
+data Bar
+  = forall x y.
+    Bar x y x y
+
+data Baz
+  = forall x y.
+    Baz
+      x
+      y
+      x
+      y

data/examples/declaration/data/existential-multiline.hs

@@ -3,3 +3,10 @@
 data Foo
   = forall a. MkFoo a (a -> Bool)
   | forall a. Eq a => MkBar a
+
+data Bar = forall x y.
+         Bar x y x y
+
+data Baz = forall x y.
+         Baz x y x
+             y

data/examples/other/multiline-forall-out.hs

@@ -12,8 +12,7 @@ data D
       )
       (x :: *)
       (y :: *).
-    D
-      (f x y)
+    D (f x y)
 
 data G where
   G ::

src/Ormolu/Printer/Meat/Declaration/Data.hs

@@ -124,18 +124,19 @@ p_conDecl = \case
         p_hsType (unLoc con_res_ty)
   ConDeclH98 {..} -> do
     mapM_ (p_hsDocString Pipe True) con_doc
-    let conDeclSpn =
-          [getLoc con_name]
-            <> [getLoc con_forall]
+    let conDeclWithContextSpn =
+          [getLoc con_forall]
             <> fmap getLoc con_ex_tvs
             <> maybeToList (fmap getLoc con_mb_cxt)
-            <> conArgsSpans con_args
-    switchLayout conDeclSpn $ do
+            <> conDeclSpn
+        conDeclSpn =
+          getLoc con_name : conArgsSpans con_args
+    switchLayout conDeclWithContextSpn $ do
       when (unLoc con_forall) $ do
         p_forallBndrs ForallInvis p_hsTyVarBndr con_ex_tvs
         breakpoint
       forM_ con_mb_cxt p_lhsContext
-      case con_args of
+      switchLayout conDeclSpn $ case con_args of
         PrefixCon xs -> do
           p_rdrName con_name
           unless (null xs) breakpoint