set compatiblity flag correctly even after incremental build

fixes webpack#5510

lib/optimize/ConcatenatedModule.js

@@ -352,7 +352,6 @@ class ConcatenatedModule extends Module {
 							internalNames: new Map(),
 							exportMap: exportMap,
 							reexportMap: reexportMap,
-							needCompatibilityFlag: false,
 							hasNamespaceObject: false,
 							namespaceObjectSource: null
 						};
@@ -576,7 +575,8 @@ class ConcatenatedModule extends Module {
 		const result = new ConcatSource();
 
 		// add harmony compatibility flag (must be first because of possible circular dependencies)
-		if(moduleToInfoMap.get(this.rootModule).needCompatibilityFlag) {
+		const usedExports = this.rootModule.usedExports;
+		if(usedExports === true) {
 			result.add(`Object.defineProperty(${this.exportsArgument || "exports"}, "__esModule", { value: true });\n`);
 		}
 
@@ -807,9 +807,7 @@ class HarmonyCompatibilityDependencyConcatenatedTemplate {
 	}
 
 	apply(dep, source, outputOptions, requestShortener, dependencyTemplates) {
-		if(dep.originModule === this.rootModule) {
-			this.modulesMap.get(this.rootModule).needCompatibilityFlag = true;
-		}
+		// do nothing
 	}
 }
 

test/hotCases/concat/reload-compat-flag/a.js

@@ -0,0 +1 @@
+export default 1;

test/hotCases/concat/reload-compat-flag/index.js

@@ -0,0 +1,17 @@
+var x = require("./module");
+
+it("should allow to hot replace modules in a ConcatenatedModule", function(done) {
+	x.should.be.eql({
+		default: "ok1",
+		__esModule: true
+	});
+	module.hot.accept("./module", function() {
+		x = require("./module");
+		x.should.be.eql({
+			default: "ok2",
+			__esModule: true
+		});
+		done();
+	});
+	NEXT(require("../../update")(done));
+});

test/hotCases/concat/reload-compat-flag/module.js

@@ -0,0 +1,7 @@
+import "./a";
+
+export default "ok1";
+---
+import "./a";
+
+export default "ok2";

test/hotCases/concat/reload-compat-flag/webpack.config.js

@@ -0,0 +1,9 @@
+"use strict";
+
+const webpack = require("../../../../");
+
+module.exports = {
+	plugins: [
+		new webpack.optimize.ModuleConcatenationPlugin()
+	]
+};