Put multiline when condition on the next line. Fixes fsprojects#1320.

src/Fantomas.Tests/PatternMatchingTests.fs

@@ -1060,3 +1060,36 @@ let rec (|DoExprAttributesL|_|) =
     | Attributes _ as x :: ys -> Some([ x ], ys)
     | _ -> None
 """
+
+[<Test>]
+let ``multiline when condition, 1320`` () =
+    formatSourceString
+        false
+        """
+module Foo =
+    module Bar =
+            let buildUsage argInfos =
+                match v.IsMember, v.IsInstanceMember, v.LogicalName, v.DisplayName with
+                // Ordinary functions or values
+                | false, _, _, name when
+                    not (hasAttribute<RequireQualifiedAccessAttribute> v.ApparentEnclosingEntity.Attributes) ->
+                    name + " " + parArgs
+                // Ordinary static members or things (?) that require fully qualified access
+                | _, _, _, name -> name + parArgs
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+module Foo =
+    module Bar =
+        let buildUsage argInfos =
+            match v.IsMember, v.IsInstanceMember, v.LogicalName, v.DisplayName with
+            // Ordinary functions or values
+            | false, _, _, name when
+                not (hasAttribute<RequireQualifiedAccessAttribute> v.ApparentEnclosingEntity.Attributes) ->
+                  name + " " + parArgs
+            // Ordinary static members or things (?) that require fully qualified access
+            | _, _, _, name -> name + parArgs
+"""

src/Fantomas/CodePrinter.fs

@@ -4230,7 +4230,11 @@ and genClause astContext hasBar (Clause (p, e, eo)) =
         mkRange "arrowRange" p.Range.End e.Range.Start
 
     let body =
-        optPre (!- " when ") sepNone eo (genExpr astContext)
+        optPre
+            (!- " when")
+            sepNone
+            eo
+            (fun e -> sepSpaceOrIndentAndNlnIfExpressionExceedsPageWidth (genExpr astContext e))
         +> sepArrow
         +> leaveNodeTokenByName arrowRange RARROW
         +> clauseBody e