-- in.cue --
X: {
	#Simple: {
		ref: string
	}

	#Complex: {
		{
			ref: string
		} | {
			local: string
		}
	}

	var: "XXX"

	// Validate(concrete) after Fill() on Complex.ref from a "static" key will succeed
	"test_1": #Complex

	// Validate(concrete) after Fill() on "#Simple.ref" from a "dynamic" key will succeed
	"test_2_\(var)": #Simple
	
	// Validate(concrete) after Fill() on Complex.ref from a "dynamic" key will fail
	"test_3_\(var)": #Complex

	test_3_XXX: ref: "FOO"
}

Z: X.test_3_XXX.ref
-- out/eval --
(struct){
  X: (struct){
    #Simple: (#struct){
      ref: (string){ string }
    }
    #Complex: (struct){ |((#struct){
        ref: (string){ string }
      }, (#struct){
        local: (string){ string }
      }) }
    var: (string){ "XXX" }
    test_1: (struct){ |((#struct){
        ref: (string){ string }
      }, (#struct){
        local: (string){ string }
      }) }
    test_3_XXX: (#struct){
      ref: (string){ "FOO" }
    }
    test_2_XXX: (#struct){
      ref: (string){ string }
    }
  }
  Z: (string){ "FOO" }
}
-- out/compile --
--- in.cue
{
  X: {
    #Simple: {
      ref: string
    }
    #Complex: {
      ({
        ref: string
      }|{
        local: string
      })
    }
    var: "XXX"
    test_1: 〈0;#Complex〉
    "test_2_\(〈0;var〉)": 〈0;#Simple〉
    "test_3_\(〈0;var〉)": 〈0;#Complex〉
    test_3_XXX: {
      ref: "FOO"
    }
  }
  Z: 〈0;X〉.test_3_XXX.ref
}