These should all be an incomplete errors.

-- in.cue --
s: string

e1: s + s
e2: >"bar" & s // TODO
e3: >s & "foo" // TODO

e4: >e1 & s
e5: <e5 & s

E: {
  a: c-b
  b: c-a
  c: a+b & >=5
}

// Issue #129
permanentlyIncompleteOperands: {
    a: string + ":" + string
    a: "golang/go:1.13.5"
}

-- out/eval --
Errors:
permanentlyIncompleteOperands.a: invalid operand string ('+' requires concrete value):
    ./in.cue:18:8

Result:
(_|_){
  // [eval]
  s: (string){ string }
  e1: (_|_){
    // [incomplete] e1: non-concrete value string in operand to +:
    //     ./in.cue:3:5
  }
  e2: (string){ >"bar" }
  e3: (string){ "foo" }
  e4: (_|_){
    // [incomplete] e1: non-concrete value string in operand to +:
    //     ./in.cue:3:5
  }
  e5: (_|_){
    // [cycle] cycle error
  }
  E: (struct){
    a: (_|_){
      // [cycle] cycle error
    }
    b: (_|_){
      // [cycle] cycle error
    }
    c: (_|_){
      // [cycle] cycle error
    }
  }
  permanentlyIncompleteOperands: (_|_){
    // [eval]
    a: (_|_){
      // [eval] permanentlyIncompleteOperands.a: invalid operand string ('+' requires concrete value):
      //     ./in.cue:18:8
    }
  }
}
-- out/compile --
--- in.cue
{
  s: string
  e1: (〈0;s〉 + 〈0;s〉)
  e2: (>"bar" & 〈0;s〉)
  e3: (>〈0;s〉 & "foo")
  e4: (>〈0;e1〉 & 〈0;s〉)
  e5: (<〈0;e5〉 & 〈0;s〉)
  E: {
    a: (〈0;c〉 - 〈0;b〉)
    b: (〈0;c〉 - 〈0;a〉)
    c: ((〈0;a〉 + 〈0;b〉) & >=5)
  }
  permanentlyIncompleteOperands: {
    a: ((string + ":") + string)
    a: "golang/go:1.13.5"
  }
}