[core-tracing] Fix the return type of withSpan

sdk/core/core-tracing/review/core-tracing.api.md

@@ -30,6 +30,11 @@ export interface OperationTracingOptions {
     tracingContext?: TracingContext;
 }
 
+// @public
+export type Resolved<T> = T extends {
+    then(onfulfilled: infer F): any;
+} ? F extends (value: infer V) => any ? Resolved<V> : never : T;
+
 // @public
 export type SpanStatus = SpanStatusSuccess | SpanStatusError;
 
@@ -57,7 +62,7 @@ export interface TracingClient {
     withContext<CallbackArgs extends unknown[], Callback extends (...args: CallbackArgs) => ReturnType<Callback>>(context: TracingContext, callback: Callback, ...callbackArgs: CallbackArgs): ReturnType<Callback>;
     withSpan<Options extends {
         tracingOptions?: OperationTracingOptions;
-    }, Callback extends (updatedOptions: Options, span: Omit<TracingSpan, "end">) => ReturnType<Callback>>(name: string, operationOptions: Options, callback: Callback, spanOptions?: TracingSpanOptions): Promise<ReturnType<Callback>>;
+    }, Callback extends (updatedOptions: Options, span: Omit<TracingSpan, "end">) => ReturnType<Callback>>(name: string, operationOptions: Options, callback: Callback, spanOptions?: TracingSpanOptions): Promise<Resolved<ReturnType<Callback>>>;
 }
 
 // @public

sdk/core/core-tracing/src/index.ts

@@ -5,6 +5,7 @@ export {
   Instrumenter,
   InstrumenterSpanOptions,
   OperationTracingOptions,
+  Resolved,
   SpanStatus,
   SpanStatusError,
   SpanStatusSuccess,

sdk/core/core-tracing/src/interfaces.ts

@@ -1,6 +1,16 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT license.
 
+/**
+ * A narrower version of TypeScript 4.5's Awaited type which Recursively
+ * unwraps the "awaited type", emulating the behavior of `await`.
+ */
+export type Resolved<T> = T extends { then(onfulfilled: infer F): any } // `await` only unwraps object types with a callable `then`. Non-object types are not unwrapped
+  ? F extends (value: infer V) => any // if the argument to `then` is callable, extracts the first argument
+    ? Resolved<V> // recursively unwrap the value
+    : never // the argument to `then` was not callable
+  : T; // non-object or non-thenable
+
 /**
  * Represents a client that can integrate with the currently configured {@link Instrumenter}.
  *
@@ -12,6 +22,8 @@ export interface TracingClient {
    *
    * This is the primary interface for using Tracing and will handle error recording as well as setting the status on the span.
    *
+   * Both synchronous and asynchronous functions will be awaited in order to reflect the result of the callback on the span.
+   *
    * Example:
    *
    * ```ts
@@ -32,7 +44,7 @@ export interface TracingClient {
     operationOptions: Options,
     callback: Callback,
     spanOptions?: TracingSpanOptions
-  ): Promise<ReturnType<Callback>>;
+  ): Promise<Resolved<ReturnType<Callback>>>;
   /**
    * Starts a given span but does not set it as the active span.
    *

sdk/core/core-tracing/src/tracingClient.ts

@@ -3,6 +3,7 @@
 
 import {
   OperationTracingOptions,
+  Resolved,
   TracingClient,
   TracingClientOptions,
   TracingContext,
@@ -64,14 +65,14 @@ export function createTracingClient(options: TracingClientOptions): TracingClien
     operationOptions: Options,
     callback: Callback,
     spanOptions?: TracingSpanOptions
-  ): Promise<ReturnType<Callback>> {
+  ): Promise<Resolved<ReturnType<Callback>>> {
     const { span, updatedOptions } = startSpan(name, operationOptions, spanOptions);
     try {
       const result = await withContext(updatedOptions.tracingOptions!.tracingContext!, () =>
         Promise.resolve(callback(updatedOptions, span))
       );
       span.setStatus({ status: "success" });
-      return result;
+      return result as ReturnType<typeof withSpan>;
     } catch (err) {
       span.setStatus({ status: "error", error: err });
       throw err;