uid function not working in upsert for variables with values

[campoy]

What version of Dgraph are you using?

master

Have you tried reproducing the issue with the latest release?

yes

What is the hardware spec (RAM, OS)?

n/a

Steps to reproduce the issue (command/config used to run Dgraph).

Create a counter value with the following mutation:

{
  set {
    _:p <value> "0"^^<xs:int> .
  }
}

Then increment its value with the given upsert block:

upsert {
  query {
    q(func: has(value)) {
      value as value
      inc as math(value+1)
    }
  }
  
  mutation {
    set {
      uid(inc) <value> val(inc) .
    }
  }
}

Expected behaviour and actual result.

The upsert above should return the previous counter value (0) and the value of inc (1), as follows:

{
  "data": {
    "code": "Success",
    "message": "Done",
    "queries": {
      "q": [
        {
          "value": 0,
          "val(inc)": 1
        }
      ]
    },
    "uids": {}
  },
  [...]
}

And a query should return the new value is 1.

 {
    q(func: has(value)) {
      value
    }
}

{
  "data": {
    "q": [
      {
        "value": 1
      }
    ]
  },
  [...]
}

Encountered behavior

The upsert returns the same value in this case, but no operation is actually performed.
We can see this using the previous query:

 {
    q(func: has(value)) {
      value
    }
}

{
  "data": {
    "q": [
      {
        "value": 0
      }
    ]
  },
  [...]
}

Even worse, if we change the upsert to set the value of value to 42 instead of using val(inc) a new UID will be created.

Running this upsert:

upsert {
  query {
    q(func: has(value)) {
      value as value
      inc as math(value+1)
    }
  }
  
  mutation {
    set {
      uid(inc) <value> "42" .
    }
  }
}

{
  "data": {
    "code": "Success",
    "message": "Done",
    "queries": {
      "q": [
        {
          "value": 0,
          "val(inc)": 1
        }
      ]
    },
    "uids": {
      "uid(inc)": "0x2"
    }
  },
  [...]
}

Instead of updating the previous value it created a new UID, as shown in the query below.

 {
    q(func: has(value)) {
      value
    }
}

{
  "data": {
    "q": [
      {
        "value": 0
      },
      {
        "value": 42
      }
    ]
  },
  [...]
}