Bugfix: In CE's with .BindReturn(..), return inside if-then (without else) stopped working with F# 5.0.

src/Compiler/Checking/CheckComputationExpressions.fs

@@ -1672,6 +1672,9 @@ let TcComputationExpression (cenv: cenv) env (overallTy: OverallTy) tpenv (mWhol
                 let bindCall = mkSynCall bindName bindRange (bindArgs @ [consumeExpr])
                 translatedCtxt (bindCall |> addBindDebugPoint))
 
+    /// This function is for desugaring into .Bind{N}Return calls if possible
+    /// The outer option indicates if .BindReturn is possible. When it returns None, .BindReturn cannot be used
+    /// The inner option indicates if a custom operation is involved inside
     and convertSimpleReturnToExpr varSpace innerComp =
         match innerComp with 
         | SynExpr.YieldOrReturn ((false, _), returnExpr, m) ->
@@ -1697,7 +1700,8 @@ let TcComputationExpression (cenv: cenv) env (overallTy: OverallTy) tpenv (mWhol
             | Some (thenExpr, None) ->
             let elseExprOptOpt  = 
                 match elseCompOpt with 
-                | None -> Some None 
+                // When we are missing an 'else' part alltogether in case of 'if cond then return exp', we fallback from BindReturn into regular Bind+Return
+                | None -> None 
                 | Some elseComp -> 
                     match convertSimpleReturnToExpr varSpace elseComp with
                     | None -> None // failure

tests/FSharp.Compiler.ComponentTests/Language/ComputationExpressionTests.fs

@@ -92,4 +92,47 @@ let x = lb {1; 2; if true then 3;}
         |> compile
         |> shouldFail
         |> withSingleDiagnostic (Error 708, Line 10, Col 19, Line 10, Col 31, "This control construct may only be used if the computation expression builder defines a 'Zero' method")
-        |> ignore
+        |> ignore
+
+    [<Theory>]
+    [<InlineData("preview","BindReturn")>]
+    [<InlineData("preview","BindReturn")>]
+    [<InlineData("preview","WithoutBindReturn")>]
+    [<InlineData("4.7","BindReturn")>]   
+    [<InlineData("4.7","WithoutBindReturn")>]  
+    let ``A CE with BindReturn and Zero can omit else in an if-then return`` (langVersion, bindReturnName) = 
+        let code = $"""
+type Builder () =
+    member inline __.Return (x: 'T) = Seq.singleton x
+    member inline __.Bind (p: seq<'T>, rest: 'T->seq<'U>) = Seq.collect rest p
+    member inline __.Zero () = Seq.empty
+    member inline __.%s{bindReturnName}  (x : seq<'T>, f: 'T -> 'U)  = Seq.map f x
+
+let seqbuilder= new Builder ()
+
+let _pythags = seqbuilder {{
+  let! z = seq [5;10]  
+  if (z > 6) then return (z,z) }} """
+        code
+        |> FSharp
+        |> withLangVersion langVersion
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>] 
+    let ``A CE with BindReturn and Zero can work without Return if flow control is not used`` () = 
+        let code = $"""
+type Builder () =
+    member inline __.Bind (p: seq<'T>, rest: 'T->seq<'U>) = Seq.collect rest p
+    //member inline __.Zero () = Seq.empty
+    member inline __.BindReturn  (x : seq<'T>, f: 'T -> 'U)  = Seq.map f x
+
+let seqbuilder= new Builder ()
+
+let _pythags = seqbuilder {{
+  let! z = seq [5;10]  
+  return (z,z) }} """
+        code
+        |> FSharp     
+        |> typecheck
+        |> shouldSucceed