Fix custom extension compilation without disabling Babel

Fixes #1836.

api.js

@@ -232,11 +232,14 @@ class Api extends Emittery {
 			filename => {
 				throw new Error(`Cannot apply full precompilation, possible bad usage: ${filename}`);
 			};
-		const precompileEnhancementsOnly = compileEnhancements && this.options.extensions.enhancementsOnly.length > 0 ?
-			babelPipeline.build(projectDir, cacheDir, null, compileEnhancements) :
-			filename => {
-				throw new Error(`Cannot apply enhancement-only precompilation, possible bad usage: ${filename}`);
-			};
+		let precompileEnhancementsOnly = () => null;
+		if (compileEnhancements) {
+			precompileEnhancementsOnly = this.options.extensions.enhancementsOnly.length > 0 ?
+				babelPipeline.build(projectDir, cacheDir, null, compileEnhancements) :
+				filename => {
+					throw new Error(`Cannot apply enhancement-only precompilation, possible bad usage: ${filename}`);
+				};
+		}
 
 		this._precompiler = {
 			cacheDir,

package.json

@@ -161,6 +161,7 @@
 		"tap": "^12.0.1",
 		"temp-write": "^3.4.0",
 		"touch": "^3.1.0",
+		"ts-node": "^6.1.1",
 		"typescript": "^2.8.3",
 		"xo": "^0.21.1",
 		"zen-observable": "^0.8.8"

test/fixture/ts-node/package.json

@@ -0,0 +1,7 @@
+{
+	"ava": {
+		"compileEnhancements": false,
+		"extensions": ["ts"],
+		"require": ["ts-node/register"]
+	}
+}

test/fixture/ts-node/test.ts

@@ -0,0 +1,5 @@
+import test from '../../../';
+
+test('pass', t => {
+	t.pass();
+});

test/integration/compilation.js

@@ -68,3 +68,10 @@ test('workers load compiled helpers if in the require configuration', t => {
 		t.end();
 	});
 });
+
+test('skips babel compilation for custom extensions, with disabled enhancement compilation', t => {
+	execCli(['test.ts'], {dirname: 'fixture/ts-node'}, err => {
+		t.ifError(err);
+		t.end();
+	});
+});