BridgeJS: fix codegen for async + throws exported methods (#691)

Author: diederich

Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift

@@ -319,8 +319,19 @@ public class ExportSwift {
         func render(abiName: String) -> DeclSyntax {
             let body: CodeBlockItemListSyntax
             if effects.isAsync {
+                // Explicit closure type annotation needed when throws is present
+                // so Swift infers throws(JSException) instead of throws(any Error)
+                // See: https://github.com/swiftlang/swift/issues/76165
+                let closureHead: String
+                if effects.isThrows {
+                    let hasReturn = self.body.contains { $0.description.contains("return ") }
+                    let ret = hasReturn ? " -> JSValue" : ""
+                    closureHead = " () async throws(JSException)\(ret) in"
+                } else {
+                    closureHead = ""
+                }
                 body = """
-                    let ret = JSPromise.async {
+                    let ret = JSPromise.async {\(raw: closureHead)
                         \(CodeBlockItemListSyntax(self.body))
                     }.jsObject
                     return ret.bridgeJSLowerReturn()