Take amount of lambda parameters into account when finding the lambda…

… body. Fixes fsprojects#1782. (fsprojects#1783)

src/Fantomas.Tests/LambdaTests.fs

@@ -880,3 +880,20 @@ module Lifecycle =
                         reg.Consume User.handleGetSessionUserIdRequest)
         }
 """
+
+[<Test>]
+let ``return lambda from lambda, 1782`` () =
+    formatSourceString
+        false
+        """
+let x =
+    fun _ ->
+        fun _ -> "hello"
+"""
+        config
+    |> prepend newline
+    |> should
+        equal
+        """
+let x = fun _ -> fun _ -> "hello"
+"""

src/Fantomas/SourceParser.fs

@@ -1246,14 +1246,19 @@ let (|Clause|) (SynMatchClause.Clause (p, eo, e, _, _)) = (p, e, eo)
 let (|Lambda|_|) =
     function
     | SynExpr.Lambda (_, _, _, _, Some (pats, body), range) ->
+        let maxDepth = List.length pats
         // find the body expression from the last lambda
-        let rec visit (e: SynExpr) : SynExpr =
-            match e with
-            | SynExpr.Match (matchSeqPoint = NoDebugPointAtInvisibleBinding; clauses = [ Clause (_, expr, _) ])
-            | SynExpr.Lambda (_, _, _, SynExpr.Match(clauses = [ Clause (_, expr, _) ]), _, _) -> visit expr
-            | _ -> e
+        let rec visit (currentDepth: int) (e: SynExpr) : SynExpr =
+            if currentDepth < maxDepth then
+                match e with
+                | SynExpr.Match (matchSeqPoint = NoDebugPointAtInvisibleBinding; clauses = [ Clause (_, expr, _) ])
+                | SynExpr.Lambda (_, _, _, SynExpr.Match(clauses = [ Clause (_, expr, _) ]), _, _) ->
+                    visit (currentDepth + 1) expr
+                | _ -> e
+            else
+                e
 
-        Some(pats, visit body, range)
+        Some(pats, visit 0 body, range)
     | _ -> None
 
 // Type definitions