Core: Warn when a module callback has a promise as a return value

Fixes qunitjs#1600.

src/module.js

@@ -83,7 +83,14 @@ function processModule( name, options, executeNow, modifiers = {} ) {
 	if ( objectType( executeNow ) === "function" ) {
 		moduleStack.push( module );
 		config.currentModule = module;
-		executeNow.call( module.testEnvironment, moduleFns );
+
+		const cbReturnValue = executeNow.call( module.testEnvironment, moduleFns );
+		if ( cbReturnValue != null && objectType( cbReturnValue.then ) === "function" ) {
+			Logger.warn( "Returning a promise from a module callback is not supported. " +
+				"Instead, use hooks for async behavior. " +
+				"This will become an error in QUnit 3.0." );
+		}
+
 		moduleStack.pop();
 		module = module.parentModule || currentModule;
 	}

test/cli/fixtures/async-module-warning/promise-test.js

@@ -0,0 +1,6 @@
+QUnit.module( "module manually returning a promise", function() {
+	QUnit.test( "has a test", function( assert ) {
+		assert.true( true );
+	} );
+	return Promise.resolve( 1 );
+} );

test/cli/fixtures/async-module-warning/test.js

@@ -0,0 +1,10 @@
+// eslint-disable-next-line qunit/no-async-module-callbacks
+QUnit.module( "module with async callback", async function() {
+	await Promise.resolve( 1 );
+
+	QUnit.test( "has a test", function( assert ) {
+		assert.true( true );
+	} );
+} );
+
+QUnit.module( "resulting parent module" );

test/cli/fixtures/expected/tap-outputs.js

@@ -339,6 +339,24 @@ ok 1 module providing hooks > module not providing hooks > has a test
 # pass 1
 # skip 0
 # todo 0
+# fail 0`,
+
+	"qunit async-module-warning/test.js":
+`TAP version 13
+ok 1 resulting parent module > has a test
+1..1
+# pass 1
+# skip 0
+# todo 0
+# fail 0`,
+
+	"qunit async-module-warning/promise-test.js":
+`TAP version 13
+ok 1 module manually returning a promise > has a test
+1..1
+# pass 1
+# skip 0
+# todo 0
 # fail 0`,
 
 	"qunit config-module.js":

test/cli/main.js

@@ -498,6 +498,24 @@ QUnit.module( "CLI Main", () => {
 		assert.equal( execution.stdout, expectedOutput[ command ] );
 	} );
 
+	QUnit.test( "warns about unsupported async module callback", async assert => {
+		const command = "qunit async-module-warning/test.js";
+		const execution = await execute( command );
+
+		assert.equal( execution.code, 0 );
+		assert.equal( execution.stderr, "Returning a promise from a module callback is not supported. Instead, use hooks for async behavior. This will become an error in QUnit 3.0.", "The warning shows" );
+		assert.equal( execution.stdout, expectedOutput[ command ] );
+	} );
+
+	QUnit.test( "warns about unsupported promise return value from module", async assert => {
+		const command = "qunit async-module-warning/promise-test.js";
+		const execution = await execute( command );
+
+		assert.equal( execution.code, 0 );
+		assert.equal( execution.stderr, "Returning a promise from a module callback is not supported. Instead, use hooks for async behavior. This will become an error in QUnit 3.0.", "The warning shows" );
+		assert.equal( execution.stdout, expectedOutput[ command ] );
+	} );
+
 	QUnit.module( "assert.expect failing conditions", () => {
 		QUnit.test( "mismatched expected assertions", async assert => {
 			const command = "qunit assert-expect/failing-expect.js";