loader: warn when the user imports a module with a thenable namespace

A module with a thenable namespace will most likely make the import fail
so warn the user when it happens. As an example, if your entry file
imports a thenable namespace and expects to continue execution after
getting it, node will currently exit with no message.

lib/internal/modules/esm/loader.js

@@ -12,11 +12,16 @@ const defaultResolve = require('internal/modules/esm/default_resolve');
 const createDynamicModule = require(
   'internal/modules/esm/create_dynamic_module');
 const translators = require('internal/modules/esm/translators');
+const { SafeWeakSet } = require('internal/safe_globals');
 
 const FunctionBind = Function.call.bind(Function.prototype.bind);
 
 const debug = require('util').debuglog('esm');
 
+const thenableNamespaces = new SafeWeakSet();
+const thenableWarning = (x) => `The namespace of ${x} is thenable. \
+See https://mdn.io/thenable for more information.`;
+
 /* A Loader instance is used as the main entry point for loading ES modules.
  * Currently, this is a singleton -- there is only one used for loading
  * the main module and everything in its dependency graph. */
@@ -73,7 +78,14 @@ class Loader {
   async import(specifier, parent) {
     const job = await this.getModuleJob(specifier, parent);
     const module = await job.run();
-    return module.namespace();
+    const m = module.namespace();
+    if (typeof m.then === 'function' && m.then.length === 0 &&
+        !thenableNamespaces.has(m) &&
+        (!parent || !/node_modules/.test(parent))) {
+      process.emitWarning(thenableWarning(module.url), 'loader');
+      thenableNamespaces.add(m);
+    }
+    return m;
   }
 
   hook({ resolve, dynamicInstantiate }) {

lib/internal/safe_globals.js

@@ -23,4 +23,5 @@ const makeSafe = (unsafe, safe) => {
 
 exports.SafeMap = makeSafe(Map, class SafeMap extends Map {});
 exports.SafeSet = makeSafe(Set, class SafeSet extends Set {});
+exports.SafeWeakSet = makeSafe(WeakSet, class SafeWeakSet extends WeakSet {});
 exports.SafePromise = makeSafe(Promise, class SafePromise extends Promise {});

test/es-module/test-thenable-warning.js

@@ -0,0 +1,15 @@
+'use strict';
+
+// Flags: --experimental-modules
+
+const common = require('../common');
+const assert = require('assert');
+
+common.crashOnUnhandledRejection();
+
+const re = /^The namespace of .+? is thenable\. See https:\/\/mdn\.io\/thenable for more information\.$/;
+process.on('warning', common.mustCall((warning) => {
+  assert(re.test(warning.message));
+}));
+
+import('../fixtures/es-modules/thenable.mjs');

test/fixtures/es-modules/thenable.mjs

@@ -0,0 +1 @@
+export function then() {}