From 852b3dc5da113546bc090d99f9250104f526f5af Mon Sep 17 00:00:00 2001
From: Romain Marcadier <rmuller@amazon.fr>
Date: Mon, 19 Sep 2022 19:58:47 +0200
Subject: [PATCH] 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                | 23 ++++++++++-
 .../test/__snapshots__/negatives.test.js.snap |  8 ++--
 packages/jsii/test/promise.test.ts            | 40 +++++++++++++++++++
 3 files changed, 65 insertions(+), 6 deletions(-)
 create mode 100644 packages/jsii/test/promise.test.ts

diff --git a/packages/jsii/lib/assembler.ts b/packages/jsii/lib/assembler.ts
index 46816f1a36..94342d2ed0 100644
--- a/packages/jsii/lib/assembler.ts
+++ b/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;
 }
 
diff --git a/packages/jsii/test/__snapshots__/negatives.test.js.snap b/packages/jsii/test/__snapshots__/negatives.test.js.snap
index 64b86cf3fc..c0036ba4fa 100644
--- a/packages/jsii/test/__snapshots__/negatives.test.js.snap
+++ b/packages/jsii/test/__snapshots__/negatives.test.js.snap
@@ -624,19 +624,19 @@ neg.omit.2.ts:7:32 - error JSII3004: Illegal implements clause for an exported A
 `;
 
 exports[`omit.3 1`] = `
-neg.omit.3.ts:8:3 - error JSII1003: Only string-indexed map types are supported
+neg.omit.3.ts:8:10 - error JSII1003: Only string-indexed map types are supported
 
 8   bar(): Omit<FooBar, 'foo'>;
-    ~~~
+           ~~~~~~~~~~~~~~~~~~~
 
 
 `;
 
 exports[`omit.4 1`] = `
-neg.omit.4.ts:8:7 - error JSII1003: Only string-indexed map types are supported
+neg.omit.4.ts:8:13 - error JSII1003: Only string-indexed map types are supported
 
 8   bar(opts: Omit<FooBar, 'foo'>): void;
-        ~~~~
+              ~~~~~~~~~~~~~~~~~~~
 
 
 `;
diff --git a/packages/jsii/test/promise.test.ts b/packages/jsii/test/promise.test.ts
new file mode 100644
index 0000000000..bdb6255c8c
--- /dev/null
+++ b/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',
+  });
+});