fix bug where modules are put into the wrong chunk

fixes #6696

lib/optimize/SplitChunksPlugin.js

@@ -304,6 +304,8 @@ module.exports = class SplitChunksPlugin {
 													chunk => !chunk.canBeInitial()
 												)
 											: Array.from(chunkCombination);
+								// Break if minimum number of chunks is not reached
+								if (selectedChunks.length < cacheGroup.minChunks) continue;
 								// Determine name for split chunk
 								const name = cacheGroup.getName(
 									module,

test/statsCases/split-chunks-issue-6696/a.js

@@ -0,0 +1 @@
+import "x";

test/statsCases/split-chunks-issue-6696/b.js

@@ -0,0 +1 @@
+import "x";

test/statsCases/split-chunks-issue-6696/expected.txt

@@ -0,0 +1,15 @@
+Entrypoint main = vendors.js main.js
+chunk    {0} async-a.js (async-a) 32 bytes <{2}> <{3}> [rendered]
+    > ./a [3] ./index.js 2:0-47
+    [0] ./node_modules/x.js 20 bytes {0} {1} [built]
+    [1] ./a.js 12 bytes {0} [built]
+chunk    {1} async-b.js (async-b) 32 bytes <{2}> <{3}> [rendered]
+    > ./b [3] ./index.js 3:0-47
+    [0] ./node_modules/x.js 20 bytes {0} {1} [built]
+    [2] ./b.js 12 bytes {1} [built]
+chunk    {2} main.js (main) 110 bytes ={3}= >{0}< >{1}< [entry] [rendered]
+    > ./ main
+    [3] ./index.js 110 bytes {2} [built]
+chunk    {3} vendors.js (vendors) 20 bytes ={2}= >{0}< >{1}< [initial] [rendered] split chunk (cache group: vendors) (name: vendors)
+    > ./ main
+    [4] ./node_modules/y.js 20 bytes {3} [built]

test/statsCases/split-chunks-issue-6696/index.js

@@ -0,0 +1,3 @@
+import "y";
+import(/* webpackChunkName: "async-a" */ "./a");
+import(/* webpackChunkName: "async-b" */ "./b");

test/statsCases/split-chunks-issue-6696/webpack.config.js

@@ -0,0 +1,34 @@
+const stats = {
+	hash: false,
+	timings: false,
+	builtAt: false,
+	assets: false,
+	chunks: true,
+	chunkOrigins: true,
+	entrypoints: true,
+	modules: false
+};
+module.exports = {
+	name: "default",
+	mode: "production",
+	entry: {
+		main: "./"
+	},
+	output: {
+		filename: "[name].js"
+	},
+	optimization: {
+		splitChunks: {
+			cacheGroups: {
+				default: false,
+				vendors: {
+					test: /[\\/]node_modules[\\/]/,
+					chunks: "initial",
+					enforce: true,
+					name: "vendors"
+				}
+			}
+		}
+	},
+	stats
+};