From ec71b4db28de68848f200393f7657e9e80f2e4f6 Mon Sep 17 00:00:00 2001
From: ncave <777696+ncave@users.noreply.github.com>
Date: Fri, 27 Sep 2024 11:07:19 -0700
Subject: [PATCH] [TS] Fixed #3864

---
 src/Fable.Cli/CHANGELOG.md           | 1 +
 src/Fable.Transforms/Fable2Babel.fs  | 1 +
 src/fable-library-ts/AsyncBuilder.ts | 2 +-
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/Fable.Cli/CHANGELOG.md b/src/Fable.Cli/CHANGELOG.md
index 9f8d4e6ed9..5722986c2b 100644
--- a/src/Fable.Cli/CHANGELOG.md
+++ b/src/Fable.Cli/CHANGELOG.md
@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * [GH-3900](https://github.com/fable-compiler/Fable/pull/3900) [Python] Fix nuget packages with hypens in their names  (by @MangelMaxime)
 * [Rust] Uncurry field types for object expressions (by @ncave)
+* [TS] Fixed missing Async type signature (#3864) (by @MangelMaxime)
 
 ## 4.21.0 - 2024-09-19
 
diff --git a/src/Fable.Transforms/Fable2Babel.fs b/src/Fable.Transforms/Fable2Babel.fs
index eb3743c2ce..5eda90e026 100644
--- a/src/Fable.Transforms/Fable2Babel.fs
+++ b/src/Fable.Transforms/Fable2Babel.fs
@@ -757,6 +757,7 @@ module Annotation =
     // Move this to Replacements.tryEntity?
     let tryNativeOrFableLibraryInterface com ctx genArgs (ent: Fable.Entity) =
         match ent.FullName with
+        | Types.fsharpAsyncGeneric -> makeFableLibImportTypeAnnotation com ctx genArgs "AsyncBuilder" "IAsync" |> Some
         | _ when not ent.IsInterface -> None
         | Types.icollection -> makeNativeTypeAnnotation com ctx genArgs "Iterable" |> Some
         // -> makeFableLibImportTypeAnnotation com ctx [Fable.Any] "Util" "ICollection"
diff --git a/src/fable-library-ts/AsyncBuilder.ts b/src/fable-library-ts/AsyncBuilder.ts
index 7ddec8abf8..e9e569353d 100644
--- a/src/fable-library-ts/AsyncBuilder.ts
+++ b/src/fable-library-ts/AsyncBuilder.ts
@@ -155,7 +155,7 @@ export class AsyncBuilder {
     }));
   }
 
-  public Return<T>(value?: T) {
+  public Return<T>(value: T) {
     return protectedReturn(value);
   }