-- in.cue --
import "list"

issue563: {
    #MyDef: {
        name: string
        ...
    }

    _all: [
        _a,
        _b,
    ]

    _a: [...#MyDef] & [
        { name: "a" },
        { name: "b" },
        { name: "c" },
    ]

    _b: [...#MyDef] & [
        { name: "1" },
        { name: "2" },
        { name: "3" },
    ]

    output: [...#MyDef] & list.FlattenN(_all, 1)
}
-- out/list --
(struct){
  issue563: (struct){
    #MyDef: (#struct){
      name: (string){ string }
    }
    _all: (#list){
      0: (#list){
        0: (#struct){
          name: (string){ "a" }
        }
        1: (#struct){
          name: (string){ "b" }
        }
        2: (#struct){
          name: (string){ "c" }
        }
      }
      1: (#list){
        0: (#struct){
          name: (string){ "1" }
        }
        1: (#struct){
          name: (string){ "2" }
        }
        2: (#struct){
          name: (string){ "3" }
        }
      }
    }
    _a: (#list){
      0: (#struct){
        name: (string){ "a" }
      }
      1: (#struct){
        name: (string){ "b" }
      }
      2: (#struct){
        name: (string){ "c" }
      }
    }
    _b: (#list){
      0: (#struct){
        name: (string){ "1" }
      }
      1: (#struct){
        name: (string){ "2" }
      }
      2: (#struct){
        name: (string){ "3" }
      }
    }
    output: (#list){
      0: (#struct){
        name: (string){ "a" }
      }
      1: (#struct){
        name: (string){ "b" }
      }
      2: (#struct){
        name: (string){ "c" }
      }
      3: (#struct){
        name: (string){ "1" }
      }
      4: (#struct){
        name: (string){ "2" }
      }
      5: (#struct){
        name: (string){ "3" }
      }
    }
  }
}