fix(jsii): unable to return Promise<void> (#3752)

The void-check only accounted for the literal `void` type, but failed to account for the `Promise<void>` case. This is now fixed.

Fixes #51



---

By submitting this pull request, I confirm that my contribution is made under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

packages/jsii/lib/assembler.ts

@@ -2222,7 +2222,11 @@ export class Assembler implements Emitter {
         protected: _isProtected(symbol) || undefined,
         returns: _isVoid(returnType)
           ? undefined
-          : this._optionalValue(returnType, declaration.name, 'return type'),
+          : this._optionalValue(
+              returnType,
+              declaration.type ?? declaration.name,
+              'return type',
+            ),
         async: _isPromise(returnType) || undefined,
         static: _isStatic(symbol) || undefined,
         locationInModule: this.declarationLocation(declaration),
@@ -2435,7 +2439,7 @@ export class Assembler implements Emitter {
       {
         ...this._optionalValue(
           this._typeChecker.getTypeAtLocation(paramDeclaration),
-          paramDeclaration.name,
+          paramDeclaration.type ?? paramDeclaration.name,
           'parameter type',
         ),
         name: paramSymbol.name,
@@ -2969,7 +2973,22 @@ function _isStatic(symbol: ts.Symbol): boolean {
   );
 }
 
+/**
+ * Determines whether a given type is void or Promise<void>.
+ *
+ * @param type the tested type
+ *
+ * @returns `true` if the type is void or Promise<void>
+ */
 function _isVoid(type: ts.Type): boolean {
+  if (_isPromise(type)) {
+    const typeRef = type as ts.TypeReference;
+    return (
+      typeRef.typeArguments != null &&
+      typeRef.typeArguments.length === 1 &&
+      _isVoid(typeRef.typeArguments[0])
+    );
+  }
   return (type.flags & ts.TypeFlags.Void) !== 0;
 }
 

packages/jsii/test/promise.test.ts

@@ -0,0 +1,40 @@
+import { sourceToAssemblyHelper } from '../lib';
+
+// ----------------------------------------------------------------------
+test('Promise<void> is a valid return type', () => {
+  const assembly = sourceToAssemblyHelper(`
+    export class PromiseMaker {
+      public static staticPromise(): Promise<void> {
+        return Promise.resolve();
+      }
+
+      public instancePromise(): Promise<void> {
+        return Promise.resolve();
+      }
+
+      private constructor() {}
+    }
+  `);
+
+  expect(assembly.types!['testpkg.PromiseMaker']).toEqual({
+    assembly: 'testpkg',
+    fqn: 'testpkg.PromiseMaker',
+    kind: 'class',
+    methods: [
+      {
+        async: true,
+        locationInModule: { filename: 'index.ts', line: 3 },
+        name: 'staticPromise',
+        static: true,
+      },
+      {
+        async: true,
+        locationInModule: { filename: 'index.ts', line: 7 },
+        name: 'instancePromise',
+      },
+    ],
+    locationInModule: { filename: 'index.ts', line: 2 },
+    name: 'PromiseMaker',
+    symbolId: 'index:PromiseMaker',
+  });
+});