Keep final do! expressions on the Bind path

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -97,6 +97,7 @@
 * Fix signature generation: `private` keyword placement for prefix-style type abbreviations. ([Issue #15560](https://github.com/dotnet/fsharp/issues/15560), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix signature generation: missing `[<Class>]` attribute for types without visible constructors. ([Issue #16531](https://github.com/dotnet/fsharp/issues/16531), [PR #19586](https://github.com/dotnet/fsharp/pull/19586))
 * Fix F# exception serialization now preserves fields (gated behind `--langversion:11`). (Issue [#878](https://github.com/dotnet/fsharp/issues/878), [PR #19342](https://github.com/dotnet/fsharp/pull/19342), [PR #19746](https://github.com/dotnet/fsharp/pull/19746))
+* Fix computation expressions desugaring final-position `do!` as `return!`/`yield!`; it now uses `Bind` while explicit `return!`/`yield!` retain final methods. ([Issue #20448](https://github.com/dotnet/fsharp/issues/20448), [PR #20449](https://github.com/dotnet/fsharp/pull/20449))
 * Fix methods being tagged as `Member` instead of `Method` in tooltips. ([Issue #10540](https://github.com/dotnet/fsharp/issues/10540), [PR #19507](https://github.com/dotnet/fsharp/pull/19507))
 * Fix Debug-mode compilation when mixing resumable and standard computation expressions. ([Issue #19625](https://github.com/dotnet/fsharp/issues/19625), [PR #19630](https://github.com/dotnet/fsharp/pull/19630))
 * IlxGen: fix missing CompilationMapping attribute for generic values ([PR #19643](https://github.com/dotnet/fsharp/pull/19643))

src/Compiler/Checking/Expressions/CheckComputationExpressions.fs

@@ -2878,22 +2878,6 @@ and TranslateComputationExpression (ceenv: ComputationExpressionContext<'a>) fir
         | None ->
             // This only occurs in final position in a sequence
             match comp with
-            // "do! expr;" in tail call position is treated as { return! expr } when ReturnFromFinal is provided
-            | SynExpr.DoBang(rhsExpr, m, _) when ceenv.tailCall && (hasBuilderMethod ceenv m "ReturnFromFinal") ->
-                let returnFrom =
-                    // Flags indicate isTrueYield, isTrueReturn
-                    SynExpr.YieldOrReturnFrom((false, true), rhsExpr, m, SynExprYieldOrReturnFromTrivia.Zero)
-
-                TranslateComputationExpression ceenv CompExprTranslationPass.Initial q varSpace returnFrom translatedCtxt
-
-            // "do! expr;" in tail call position is treated as { yield! expr } when YieldFromFinal is provided
-            | SynExpr.DoBang(rhsExpr, m, _) when ceenv.tailCall && (hasBuilderMethod ceenv m "YieldFromFinal") ->
-                let returnFrom =
-                    // Flags indicate isTrueYield, isTrueReturn
-                    SynExpr.YieldOrReturnFrom((true, false), rhsExpr, m, SynExprYieldOrReturnFromTrivia.Zero)
-
-                TranslateComputationExpression ceenv CompExprTranslationPass.Initial q varSpace returnFrom translatedCtxt
-
             // "do! expr;" in final position is treated as { let! () = expr in return () } when Return is provided (and no Zero with Default attribute is available) or as { let! () = expr in zero } otherwise
             | SynExpr.DoBang(expr = rhsExpr; trivia = { DoBangKeyword = m }) ->
                 let mUnit = rhsExpr.Range

tests/FSharp.Compiler.ComponentTests/Language/coroutines.fsx

@@ -267,14 +267,6 @@ let testTailcallTiny () =
 
 testTailcallTiny() |> expect 2 0
 
-let testTailcallTinyDoBang () = 
-    coroutine {
-        printfn "in testTailcallTinyDoBang, desugaring do!"
-        do! t1() // this should desugr to YieldFromFinal, because ReturnFromFinal is not provided.
-    }
-
-testTailcallTinyDoBang() |> expect 2 0
-
 let rec testTailcall (n: int) = 
     coroutine {
         if n % 10_000 = 0 then printfn $"in testTailcall, n = {n}"
@@ -431,4 +423,3 @@ let testHandlerNonTailCall () =
 
 testHandlerNonTailCall () |> expect 0 2
 
-

tests/FSharp.Compiler.ComponentTests/Language/tailcalls.fsx

@@ -37,10 +37,10 @@ do
 
 
 do 
-    let sync = SyncBuilder (expect ReturnFromFinal)
+    let sync = SyncBuilder expectNone
 
     sync {
-        printf "expect ReturnFromFinal: "
+        printf "expectNone: "
         do! sync { printfn "inner" }
     } |> run
 
@@ -154,6 +154,27 @@ do
     shouldEqual b.YieldFromCount 1
     shouldEqual b.YieldFromFinalCount 0
 
+// do! in final position should use Bind rather than a final YieldFrom method.
+type DoBangBuilder() =
+    member _.Bind(_: System.Threading.Tasks.Task, continuation: unit -> int list) = continuation()
+    member _.Yield(value: int) = [ value ]
+    member _.YieldFrom(_: int list) = []
+    member _.YieldFromFinal(_: int list) = []
+    member _.Return(_: unit) = []
+    member _.Combine(first: int list, second: unit -> int list) = first @ second ()
+    member _.Delay(computation: unit -> int list) = computation
+    member _.Run(computation: unit -> int list) = computation ()
+
+do
+    let builder = DoBangBuilder()
+    let result =
+        builder {
+            yield 1
+            do! System.Threading.Tasks.Task.CompletedTask
+        }
+
+    shouldEqual result [ 1 ]
+
 // yield! in try/finally body → YieldFrom (not tail), result correct
 do
     let b = ListBuilder()