add default cache names in multicompiler mode

lib/MultiCompiler.js

@@ -134,9 +134,9 @@ module.exports = class MultiCompiler {
 						new WebpackError(
 							`${
 								compiler.name
-									? `Compiler with name "${compiler.name}" doesn't use cache name. `
+									? `Compiler with name "${compiler.name}" doesn't use unique cache name. `
 									: ""
-							}Please set "cache.name" option.`
+							}Please set unique "cache.name" option.`
 						)
 					);
 				}

lib/config/normalization.js

@@ -130,16 +130,26 @@ const keyedNestedConfig = (value, fn, customKeys) => {
 const getNormalizedWebpackMultiCompilerOptions = options => {
 	/** @type {Array<WebpackOptions>} */
 	const resultedOptions = [];
+	/** @type {Set<string>} */
+	const cacheNames = new Set();
 
 	for (let i = 0; i < options.length; i++) {
 		const config = options[i];
 		resultedOptions.push({
 			...config,
 			cache: optionalNestedConfig(config.cache, cache => {
 				if (typeof cache === "boolean") return cache;
-				if (i === 0 || cache.type !== "filesystem" || "name" in cache)
+				if (cache.type !== "filesystem") return { ...cache };
+				const name = cache.name ? cache.name : defaultCacheName;
+				if (i === 0) {
+					cacheNames.add(name);
 					return { ...cache };
-				return { ...cache, name: `${defaultCacheName}__compiler${i + 1}__` };
+				}
+				if (cacheNames.has(name)) {
+					return { ...cache, name: `${name}__compiler${i + 1}__` };
+				}
+				cacheNames.add(cache.name);
+				return { ...cache };
 			})
 		});
 	}

test/ConfigTestCases.template.js

@@ -110,11 +110,12 @@ const describeCases = config => {
 										(options.experiments && options.experiments.outputModule
 											? ".mjs"
 											: ".js");
-								if (config.cache) {
+								if (config.cache || options.cache) {
 									options.cache = {
 										cacheDirectory,
-										name: `config-${idx}`,
-										...config.cache
+										name: options.cache ? options.cache.name : `config-${idx}`,
+										...config.cache,
+										...options.cache
 									};
 									options.infrastructureLogging = {
 										debug: true,

test/configCases/cache-filesystem/multicompiler-mode-cache-warning/warnings.js

@@ -1,4 +1,4 @@
 module.exports = [
-	/Please set "cache\.name" option/,
-	/Compiler with name "3rd compiler" doesn't use cache name/
+	/Please set unique "cache\.name" option/,
+	/Compiler with name "3rd compiler" doesn't use unique cache name/
 ];