module: do not warn when accessing __esModule of unfinished exports

lib/internal/modules/cjs/loader.js

@@ -825,13 +825,16 @@ function emitCircularRequireWarning(prop) {
 // warns when non-existend properties are accessed.
 const CircularRequirePrototypeWarningProxy = new Proxy({}, {
   get(target, prop) {
-    if (prop in target) return target[prop];
+    // Allow __esModule access in any case because it is used in the output
+    // of transpiled code to determine whether something comes from an
+    // ES module, and is not used as a regular key of `module.exports`.
+    if (prop in target || prop === '__esModule') return target[prop];
     emitCircularRequireWarning(prop);
     return undefined;
   },
 
   getOwnPropertyDescriptor(target, prop) {
-    if (ObjectPrototypeHasOwnProperty(target, prop))
+    if (ObjectPrototypeHasOwnProperty(target, prop) || prop === '__esModule')
       return ObjectGetOwnPropertyDescriptor(target, prop);
     emitCircularRequireWarning(prop);
     return undefined;

test/fixtures/cycles/warning-esm-half-transpiled-a.js

@@ -0,0 +1 @@
+require('./warning-esm-half-transpiled-b.js');

test/fixtures/cycles/warning-esm-half-transpiled-b.js

@@ -0,0 +1,2 @@
+const a = require('./warning-esm-half-transpiled-a.js');
+a.__esModule;

test/parallel/test-module-circular-dependency-warning.js

@@ -31,3 +31,10 @@ assert.strictEqual(Object.getPrototypeOf(classExport).name, 'Parent');
 const esmTranspiledExport =
   require(fixtures.path('cycles', 'warning-esm-transpiled-a.js'));
 assert.strictEqual(esmTranspiledExport.__esModule, true);
+
+// If module.exports.__esModule is being accessed but is not present, e.g.
+// because only the one of the files is a transpiled ES module, no warning
+// should be emitted.
+const halfTranspiledExport =
+  require(fixtures.path('cycles', 'warning-esm-half-transpiled-a.js'));
+assert.strictEqual(halfTranspiledExport.__esModule, undefined);