fix(template-compiler): allow comments on the root node in templates (#9408)

In SFC templates, we are allowed to add comments to the root node. If parsing with comments flag
true, we are not anymore. This ignores the root comments in case they cannot be added.

fix #9407

src/compiler/parser/index.js

@@ -377,16 +377,20 @@ export function parse (
       }
     },
     comment (text: string, start, end) {
-      const child: ASTText = {
-        type: 3,
-        text,
-        isComment: true
+      // adding anyting as a sibling to the root node is forbidden
+      // comments should still be allowed, but ignored
+      if (currentParent) {
+        const child: ASTText = {
+          type: 3,
+          text,
+          isComment: true
+        }
+        if (process.env.NODE_ENV !== 'production' && options.outputSourceRange) {
+          child.start = start
+          child.end = end
+        }
+        currentParent.children.push(child)
       }
-      if (process.env.NODE_ENV !== 'production' && options.outputSourceRange) {
-        child.start = start
-        child.end = end
-      }
-      currentParent.children.push(child)
     }
   })
   return root

test/unit/modules/compiler/parser.spec.js

@@ -786,6 +786,16 @@ describe('parser', () => {
     expect(ast.children[1].text).toBe('comment here')
   })
 
+  // #9407
+  it('should parse templates with comments anywhere', () => {
+    const options = extend({
+      comments: true
+    }, baseOptions)
+    const ast = parse(`<!--comment here--><div>123</div>`, options)
+    expect(ast.tag).toBe('div')
+    expect(ast.children.length).toBe(1)
+  })
+
   // #8103
   it('should allow CRLFs in string interpolations', () => {
     const ast = parse(`<p>{{\r\nmsg\r\n}}</p>`, baseOptions)
@@ -797,7 +807,7 @@ describe('parser', () => {
       preserveWhitespace: false
     }, baseOptions)
 
-     const ast = parse('<p>\n  Welcome to <b>Vue.js</b>    <i>world</i>  \n  <span>.\n  Have fun!\n</span></p>', options)
+    const ast = parse('<p>\n  Welcome to <b>Vue.js</b>    <i>world</i>  \n  <span>.\n  Have fun!\n</span></p>', options)
     expect(ast.tag).toBe('p')
     expect(ast.children.length).toBe(4)
     expect(ast.children[0].type).toBe(3)