fix(ssr): computed properties should pass vm as first argument in ssr (#9090)

fix #8977

src/core/instance/state.js

@@ -216,13 +216,13 @@ export function defineComputed (
   if (typeof userDef === 'function') {
     sharedPropertyDefinition.get = shouldCache
       ? createComputedGetter(key)
-      : userDef
+      : createGetterInvoker(userDef)
     sharedPropertyDefinition.set = noop
   } else {
     sharedPropertyDefinition.get = userDef.get
       ? shouldCache && userDef.cache !== false
         ? createComputedGetter(key)
-        : userDef.get
+        : createGetterInvoker(userDef.get)
       : noop
     sharedPropertyDefinition.set = userDef.set || noop
   }
@@ -253,6 +253,12 @@ function createComputedGetter (key) {
   }
 }
 
+function createGetterInvoker(fn) {
+  return function computedGetter () {
+    return fn.call(this, this)
+  }
+}
+
 function initMethods (vm: Component, methods: Object) {
   const props = vm.$options.props
   for (const key in methods) {

test/ssr/ssr-string.spec.js

@@ -1076,6 +1076,24 @@ describe('SSR: renderToString', () => {
     })
   })
 
+  // #8977
+  it('should call computed properties with vm as first argument', done => {
+    renderToString(new Vue({
+      data: {
+        firstName: 'Evan',
+        lastName: 'You'
+      },
+      computed: {
+        fullName: ({ firstName, lastName }) => `${firstName} ${lastName}`,
+      },
+      template: '<div>{{ fullName }}</div>',
+    }), (err, result) => {
+      expect(err).toBeNull()
+      expect(result).toContain('<div data-server-rendered="true">Evan You</div>')
+      done()
+    })
+  })
+
   it('return Promise', done => {
     renderToString(new Vue({
       template: `<div>{{ foo }}</div>`,