From ebea295eccf4a39177d73e93bc99f8e61df80acb Mon Sep 17 00:00:00 2001
From: Haoqun Jiang <haoqunjiang@gmail.com>
Date: Mon, 22 Jul 2019 20:38:39 +0800
Subject: [PATCH] fix: require parent template after `when` condition evaluated
 as truthy

fixes #4291
---
 packages/@vue/cli/lib/GeneratorAPI.js | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/packages/@vue/cli/lib/GeneratorAPI.js b/packages/@vue/cli/lib/GeneratorAPI.js
index 1261676d35..6aad56bd00 100644
--- a/packages/@vue/cli/lib/GeneratorAPI.js
+++ b/packages/@vue/cli/lib/GeneratorAPI.js
@@ -417,6 +417,22 @@ function renderFile (name, data, ejsOptions) {
   const parsed = yaml.loadFront(template)
   const content = parsed.__content
   let finalTemplate = content.trim() + `\n`
+
+  if (parsed.when) {
+    finalTemplate = (
+      `<%_ if (${parsed.when}) { _%>` +
+        finalTemplate +
+      `<%_ } _%>`
+    )
+
+    // use ejs.render to test the conditional expression
+    // if evaluated to falsy vaule, return early to avoid extra cost for extend expression
+    const result = ejs.render(finalTemplate, data, ejsOptions)
+    if (!result) {
+      return
+    }
+  }
+
   if (parsed.extend) {
     const extendPath = path.isAbsolute(parsed.extend)
       ? parsed.extend
@@ -437,13 +453,6 @@ function renderFile (name, data, ejsOptions) {
         finalTemplate = finalTemplate.replace(parsed.replace, content.trim())
       }
     }
-    if (parsed.when) {
-      finalTemplate = (
-        `<%_ if (${parsed.when}) { _%>` +
-          finalTemplate +
-        `<%_ } _%>`
-      )
-    }
   }
 
   return ejs.render(finalTemplate, data, ejsOptions)