swiftwasm · krodak · Jun 10, 2026 · Jun 10, 2026
diff --git a/Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift b/Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift
@@ -426,21 +426,45 @@ public class ExportSwift {
        /// A throwing async body needs an explicit closure type, otherwise Swift infers
        /// `throws(any Error)` instead of `throws(JSException)`.
        /// See: https://github.com/swiftlang/swift/issues/76165
-        private func asyncThrowsClosureHead(returnSpelling: String?) -> String {
+        private func asyncThrowsClosureHead(returnSpelling: String?, forcesCapture: Bool) -> String {
            guard effects.isThrows else { return "" }
            let returns = returnSpelling.map { " -> \($0)" } ?? ""
-            return " () async throws(JSException)\(returns) in"
+            let capture = forcesCapture ? "[__bjs_capture] " : ""
+            return " \(capture)() async throws(JSException)\(returns) in"
+        }
+
+        /// A captureless throwing async body closure lowers via `thin_to_thick_function`,
+        /// which miscompiles typed-error calls on wasm32. Forcing a capture that the body
+        /// reads turns the closure into a partial apply with a context, avoiding the
+        /// broken convention. An unread capture list entry is dropped by capture analysis,
+        /// so the body must also read the captured value.
+        /// See: https://github.com/swiftlang/swift/issues/89320
+        private var asyncThrowsBodyForcesCapture: Bool {
+            effects.isThrows && abiParameterSignatures.isEmpty && asyncHoistedBindings.isEmpty
        }

        func render(abiName: String) -> DeclSyntax {
            let body: CodeBlockItemListSyntax
            if effects.isAsync, let resolveType = asyncResolveReturnType {
                let resolveName = "Promise_resolve_\(resolveType.mangleTypeName)"
-                let closureHead = asyncThrowsClosureHead(returnSpelling: resolveType.swiftType)
+                let forcesCapture = asyncThrowsBodyForcesCapture
+                let closureHead = asyncThrowsClosureHead(
+                    returnSpelling: resolveType.swiftType,
+                    forcesCapture: forcesCapture
+                )
+                var hoistedBindings = asyncHoistedBindings
+                var bodyItems = self.body
+                if forcesCapture {
+                    hoistedBindings.append("let __bjs_capture = 0")
+                    if !bodyItems.isEmpty {
+                        bodyItems[0] = bodyItems[0].with(\.leadingTrivia, .newline)
+                    }
+                    bodyItems.insert("_ = __bjs_capture", at: 0)
+                }
                body = """
-                    \(CodeBlockItemListSyntax(asyncHoistedBindings))
+                    \(CodeBlockItemListSyntax(hoistedBindings))
                    return _bjs_makePromise(resolve: \(raw: resolveName), reject: Promise_reject) {\(raw: closureHead)
-                        \(CodeBlockItemListSyntax(self.body))
+                        \(CodeBlockItemListSyntax(bodyItems))
                    }
                    """
            } else if effects.isThrows {

diff --git a/Plugins/BridgeJS/Tests/BridgeJSToolTests/Inputs/MacroSwift/Async.swift b/Plugins/BridgeJS/Tests/BridgeJSToolTests/Inputs/MacroSwift/Async.swift
@@ -31,6 +31,10 @@
    return v
 }

+@JS func asyncThrowsZeroArg() async throws(JSException) -> String {
+    return "ok"
+}
+
 @JS func asyncCombineStructs(_ a: AsyncPoint, _ b: AsyncPoint) async -> AsyncPoint {
    return AsyncPoint(x: a.x + b.x, y: a.y + b.y)
 }

diff --git a/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSCodegenTests/Async.json b/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSCodegenTests/Async.json
@@ -283,6 +283,23 @@
          }
        }
      },
+      {
+        "abiName" : "bjs_asyncThrowsZeroArg",
+        "effects" : {
+          "isAsync" : true,
+          "isStatic" : false,
+          "isThrows" : true
+        },
+        "name" : "asyncThrowsZeroArg",
+        "parameters" : [
+
+        ],
+        "returnType" : {
+          "string" : {
+
+          }
+        }
+      },
      {
        "abiName" : "bjs_asyncCombineStructs",
        "effects" : {

diff --git a/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSCodegenTests/Async.swift b/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSCodegenTests/Async.swift
@@ -194,6 +194,20 @@ public func _bjs_asyncRoundTripStructThrows() -> Int32 {
    #endif
 }

+@_expose(wasm, "bjs_asyncThrowsZeroArg")
+@_cdecl("bjs_asyncThrowsZeroArg")
+public func _bjs_asyncThrowsZeroArg() -> Int32 {
+    #if arch(wasm32)
+    let __bjs_capture = 0
+    return _bjs_makePromise(resolve: Promise_resolve_SS, reject: Promise_reject) { [__bjs_capture] () async throws(JSException) -> String in
+        _ = __bjs_capture
+        return try await asyncThrowsZeroArg()
+    }
+    #else
+    fatalError("Only available on WebAssembly")
+    #endif
+}
+
 @_expose(wasm, "bjs_asyncCombineStructs")
 @_cdecl("bjs_asyncCombineStructs")
 public func _bjs_asyncCombineStructs() -> Int32 {

diff --git a/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSLinkTests/Async.d.ts b/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSLinkTests/Async.d.ts
@@ -34,6 +34,7 @@ export type Exports = {
    asyncRoundTripJSObject(v: any): Promise<any>;
    asyncRoundTripStruct(v: AsyncPoint): Promise<AsyncPoint>;
    asyncRoundTripStructThrows(v: AsyncPoint): Promise<AsyncPoint>;
+    asyncThrowsZeroArg(): Promise<string>;
    asyncCombineStructs(a: AsyncPoint, b: AsyncPoint): Promise<AsyncPoint>;
    asyncRoundTripEnum(v: AsyncDirectionTag): Promise<AsyncDirectionTag>;
    asyncRoundTripRawEnum(v: AsyncThemeTag): Promise<AsyncThemeTag>;

diff --git a/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSLinkTests/Async.js b/Plugins/BridgeJS/Tests/BridgeJSToolTests/__Snapshots__/BridgeJSLinkTests/Async.js
@@ -585,6 +585,18 @@ export async function createInstantiator(options, swift) {
                    }
                    return ret1;
                },
+                asyncThrowsZeroArg: function bjs_asyncThrowsZeroArg() {
+                    const ret = instance.exports.bjs_asyncThrowsZeroArg();
+                    const ret1 = swift.memory.getObject(ret);
+                    swift.memory.release(ret);
+                    if (tmpRetException) {
+                        const error = swift.memory.getObject(tmpRetException);
+                        swift.memory.release(tmpRetException);
+                        tmpRetException = undefined;
+                        throw error;
+                    }
+                    return ret1;
+                },
                asyncCombineStructs: function bjs_asyncCombineStructs(a, b) {
                    structHelpers.AsyncPoint.lower(a);
                    structHelpers.AsyncPoint.lower(b);

diff --git a/Tests/BridgeJSRuntimeTests/ExportAPITests.swift b/Tests/BridgeJSRuntimeTests/ExportAPITests.swift
@@ -96,6 +96,10 @@ struct TestError: Error {
 @JS func throwsWithSwiftHeapObjectResult() throws(JSException) -> Greeter { return Greeter(name: "Test") }
 @JS func throwsWithJSObjectResult() throws(JSException) -> JSObject { return JSObject() }

+@JS func zeroArgAsyncThrows() async throws(JSException) -> String {
+    throw JSException(JSError(message: "ZeroArgAsyncThrowsError").jsValue)
+}
+
 @JS func asyncRoundTripVoid() async -> Void { return }
 @JS func asyncRoundTripInt(v: Int) async -> Int { return v }
 @JS func asyncRoundTripFloat(v: Float) async -> Float { return v }

diff --git a/Tests/BridgeJSRuntimeTests/Generated/BridgeJS.swift b/Tests/BridgeJSRuntimeTests/Generated/BridgeJS.swift
@@ -7189,6 +7189,20 @@ public func _bjs_throwsWithJSObjectResult() -> Int32 {
    #endif
 }

+@_expose(wasm, "bjs_zeroArgAsyncThrows")
+@_cdecl("bjs_zeroArgAsyncThrows")
+public func _bjs_zeroArgAsyncThrows() -> Int32 {
+    #if arch(wasm32)
+    let __bjs_capture = 0
+    return _bjs_makePromise(resolve: Promise_resolve_SS, reject: Promise_reject) { [__bjs_capture] () async throws(JSException) -> String in
+        _ = __bjs_capture
+        return try await zeroArgAsyncThrows()
+    }
+    #else
+    fatalError("Only available on WebAssembly")
+    #endif
+}
+
 @_expose(wasm, "bjs_asyncRoundTripVoid")
 @_cdecl("bjs_asyncRoundTripVoid")
 public func _bjs_asyncRoundTripVoid() -> Int32 {
@@ -11403,6 +11417,28 @@ func _$Promise_reject(_ promise: JSObject, _ value: JSValue) throws(JSException)
    if let error = _swift_js_take_exception() { throw error }
 }

+@JSFunction func Promise_resolve_SS(_ promise: JSObject, _ value: String) throws(JSException)
+
+#if arch(wasm32)
+@_extern(wasm, module: "bjs", name: "promise_resolve_BridgeJSRuntimeTests_SS")
+fileprivate func promise_resolve_BridgeJSRuntimeTests_SS_extern(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void
+#else
+fileprivate func promise_resolve_BridgeJSRuntimeTests_SS_extern(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void {
+    fatalError("Only available on WebAssembly")
+}
+#endif
+@inline(never) fileprivate func promise_resolve_BridgeJSRuntimeTests_SS(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void {
+    return promise_resolve_BridgeJSRuntimeTests_SS_extern(promise, valueBytes, valueLength)
+}
+
+func _$Promise_resolve_SS(_ promise: JSObject, _ value: String) throws(JSException) -> Void {
+    let promiseValue = promise.bridgeJSLowerParameter()
+    value.bridgeJSWithLoweredParameter { (valueBytes, valueLength) in
+        promise_resolve_BridgeJSRuntimeTests_SS(promiseValue, valueBytes, valueLength)
+    }
+    if let error = _swift_js_take_exception() { throw error }
+}
+
 @JSFunction func Promise_resolve_y(_ promise: JSObject) throws(JSException)

 #if arch(wasm32)
@@ -11507,28 +11543,6 @@ func _$Promise_resolve_Sb(_ promise: JSObject, _ value: Bool) throws(JSException
    if let error = _swift_js_take_exception() { throw error }
 }

-@JSFunction func Promise_resolve_SS(_ promise: JSObject, _ value: String) throws(JSException)
-
-#if arch(wasm32)
-@_extern(wasm, module: "bjs", name: "promise_resolve_BridgeJSRuntimeTests_SS")
-fileprivate func promise_resolve_BridgeJSRuntimeTests_SS_extern(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void
-#else
-fileprivate func promise_resolve_BridgeJSRuntimeTests_SS_extern(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void {
-    fatalError("Only available on WebAssembly")
-}
-#endif
-@inline(never) fileprivate func promise_resolve_BridgeJSRuntimeTests_SS(_ promise: Int32, _ valueBytes: Int32, _ valueLength: Int32) -> Void {
-    return promise_resolve_BridgeJSRuntimeTests_SS_extern(promise, valueBytes, valueLength)
-}
-
-func _$Promise_resolve_SS(_ promise: JSObject, _ value: String) throws(JSException) -> Void {
-    let promiseValue = promise.bridgeJSLowerParameter()
-    value.bridgeJSWithLoweredParameter { (valueBytes, valueLength) in
-        promise_resolve_BridgeJSRuntimeTests_SS(promiseValue, valueBytes, valueLength)
-    }
-    if let error = _swift_js_take_exception() { throw error }
-}
-
 @JSFunction func Promise_resolve_7GreeterC(_ promise: JSObject, _ value: Greeter) throws(JSException)

 #if arch(wasm32)

diff --git a/Tests/BridgeJSRuntimeTests/Generated/JavaScript/BridgeJS.json b/Tests/BridgeJSRuntimeTests/Generated/JavaScript/BridgeJS.json
@@ -12171,6 +12171,23 @@
          }
        }
      },
+      {
+        "abiName" : "bjs_zeroArgAsyncThrows",
+        "effects" : {
+          "isAsync" : true,
+          "isStatic" : false,
+          "isThrows" : true
+        },
+        "name" : "zeroArgAsyncThrows",
+        "parameters" : [
+
+        ],
+        "returnType" : {
+          "string" : {
+
+          }
+        }
+      },
      {
        "abiName" : "bjs_asyncRoundTripVoid",
        "effects" : {

diff --git a/Tests/BridgeJSRuntimeTests/JavaScript/AsyncImportTests.mjs b/Tests/BridgeJSRuntimeTests/JavaScript/AsyncImportTests.mjs
@@ -64,6 +64,11 @@ export async function runAsyncWorksTests(exports) {
        (error) => error instanceof Error && error.message === "async struct failure"
    );

+    await assert.rejects(
+        () => exports.zeroArgAsyncThrows(),
+        (error) => error instanceof Error && error.message === "ZeroArgAsyncThrowsError"
+    );
+
    const richContact = {
        name: "Alice",
        age: 30,