-- in.cue --
package x

#x: {
	Name: string
}

_#x: {
	Name: string
}

x1: #x & {
	Name: "hello"
	Age:  50
}

x2: _#x & {
	Name: "hello"
	Age:  50
}
-- out/eval --
Errors:
x1: field not allowed: Age:
    ./in.cue:3:5
    ./in.cue:11:5
    ./in.cue:13:2
x2: field not allowed: Age:
    ./in.cue:7:6
    ./in.cue:16:5
    ./in.cue:18:2

Result:
(_|_){
  // [eval]
  #x: (#struct){
    Name: (string){ string }
  }
  _#x(:x): (#struct){
    Name: (string){ string }
  }
  x1: (_|_){
    // [eval]
    Name: (string){ "hello" }
    Age: (_|_){
      // [eval] x1: field not allowed: Age:
      //     ./in.cue:3:5
      //     ./in.cue:11:5
      //     ./in.cue:13:2
    }
  }
  x2: (_|_){
    // [eval]
    Name: (string){ "hello" }
    Age: (_|_){
      // [eval] x2: field not allowed: Age:
      //     ./in.cue:7:6
      //     ./in.cue:16:5
      //     ./in.cue:18:2
    }
  }
}
-- out/compile --
--- in.cue
{
  #x: {
    Name: string
  }
  _#x: {
    Name: string
  }
  x1: (〈0;#x〉 & {
    Name: "hello"
    Age: 50
  })
  x2: (〈0;_#x〉 & {
    Name: "hello"
    Age: 50
  })
}