[BUG]: Count cannot be assigned to a value variable

What version of Dgraph are you using?

Dgraph Cloud v21.03.0-92-g0c9f60156

Tell us a little more about your go-environment?

No response

Have you tried reproducing the issue with the latest release?

Yes

What is the hardware spec (RAM, CPU, OS)?

Tier: 2R
Storage: 32 GB
Dgraph Hosted: Yes
High Availability: No
SLA: 99.5%
Schema Mode: Flexible

This error can also be reproduced on a Shared Cluster (Free Tier) with a minimal schema.

What steps will reproduce the bug?

1. Deploy minimal schema

type Course {
  id: ID!
  title: String!
  chapters: [Chapter!]!
}

type Chapter {
  id: ID!
  title: String!
  sequence: Int
}

2. Perform initial DQL mutation

{
  "set": {
    "dgraph.type": "Course",
    "Course.title": "Course 1",
    "Course.chapters": [
      {
        "dgraph.type": "Chapter",
        "Chapter.title": "Chapter 1",
        "Chapter.sequence": 0
      },
      {
        "dgraph.type": "Chapter",
        "Chapter.title": "Chapter 2",
        "Chapter.sequence": 1
      }
    ]
  } 
}

3. Perform upsert DQL mutation

{
  "query": "{ qTest(func: uid(0x1)) { u as uid c as count(Course.chapters) } }",
  "mutations": [
    {
      "set": {
        "uid": "uid(u)",
        "Course.chapters": [
          {
            "dgraph.type": "Chapter",
            "Chapter.title": "Test Chapter",
            "Chapter.sequence": "val(c)"
          }
        ]
      }
    }
  ]
}

Response

{
  data: {
    code: "Success"
    message: "Done"
    queries: {
      qTest: [
        0: {
          uid:"0x1"
          count(Course.chapters): 2
        }
      ]
    }
    uids: {
      dg.3162278161.22051:"0x1"
    }
}

Expected behavior and actual result.

We would expect that Chapter.sequence of the new Chapter Test Chapter would equal 2 since count(Course.chapters) validates 2.

This behaviour is also documented in the official docs.

Additional information

Chapter.sequence of the newly created Chapter Test Chapter remains undefined.

No source files or tests are named. Reproduce the issue with the minimal Course and Chapter schema, then run the shown DQL upsert and trace how count(Course.chapters) is assigned through val(c). Done means the new Chapter's Chapter.sequence is set to 2 rather than remaining undefined.