fix: use flat config when eslintrc config does not exist

packages/config-helpers/src/define-config.js

@@ -251,6 +251,8 @@ function findPluginConfig(config, pluginConfigName) {
 	}
 
 	const directConfig = plugin.configs?.[configName];
+
+	// Prefer direct config, but fall back to flat config if available
 	if (directConfig) {
 		// Arrays are always flat configs, and non-legacy configs can be used directly
 		if (Array.isArray(directConfig) || !isLegacyConfig(directConfig)) {
@@ -261,30 +263,28 @@ function findPluginConfig(config, pluginConfigName) {
 				pluginConfigName,
 			);
 		}
+	}
 
-		// If it's a legacy config, look for the flat version
-		const flatConfig = plugin.configs?.[`flat/${configName}`];
-
-		if (
-			flatConfig &&
-			(Array.isArray(flatConfig) || !isLegacyConfig(flatConfig))
-		) {
-			return deepNormalizePluginConfig(
-				userPluginNamespace,
-				plugin,
-				flatConfig,
-				pluginConfigName,
-			);
-		}
-
-		throw new TypeError(
-			`Plugin config "${configName}" in plugin "${userPluginNamespace}" is an eslintrc config and cannot be used in this context.`,
+	// If it's a legacy config, or the config does not exist => look for the flat version
+	const flatConfig = plugin.configs?.[`flat/${configName}`];
+	if (
+		flatConfig &&
+		(Array.isArray(flatConfig) || !isLegacyConfig(flatConfig))
+	) {
+		return deepNormalizePluginConfig(
+			userPluginNamespace,
+			plugin,
+			flatConfig,
+			pluginConfigName,
 		);
 	}
 
-	throw new TypeError(
-		`Plugin config "${configName}" not found in plugin "${userPluginNamespace}".`,
-	);
+	// If we get here, then the config was either not found or is a legacy config
+	const message =
+		directConfig || flatConfig
+			? `Plugin config "${configName}" in plugin "${userPluginNamespace}" is an eslintrc config and cannot be used in this context.`
+			: `Plugin config "${configName}" not found in plugin "${userPluginNamespace}".`;
+	throw new TypeError(message);
 }
 
 /**

packages/config-helpers/tests/define-config.test.js

@@ -826,6 +826,37 @@ describe("defineConfig()", () => {
 				]);
 			});
 
+			it("should use flat config when eslintrc does not exist", () => {
+				const testPlugin = {
+					configs: {
+						"flat/recommended": {
+							rules: { "no-console": "error" },
+						},
+					},
+				};
+
+				const config = defineConfig({
+					plugins: {
+						test: testPlugin,
+					},
+					extends: ["test/recommended"],
+					rules: {
+						"no-debugger": "error",
+					},
+				});
+
+				assert.deepStrictEqual(config, [
+					{
+						name: "UserConfig[0] > test/recommended",
+						rules: { "no-console": "error" },
+					},
+					{
+						plugins: { test: testPlugin },
+						rules: { "no-debugger": "error" },
+					},
+				]);
+			});
+
 			it("should throw error when both base and flat configs are legacy", () => {
 				const testPlugin = {
 					configs: {