[TS] Fixed #3864 (#3904)

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
 

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"

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);
   }