GALA-E0092 — Copy names a field the sealed value cannot have

When it fires. Copy(field = ...) on a value of a sealed type names a field that Copy cannot set:

the field belongs to some cases but not all of them. A value whose type is the sealed type may be any of its cases, so Copy sets only a field every case has;
the value is a constructor call, Sq(2.0, "s").Copy(r = 9.0), whose case has no such field;
no case of the type has a field of that name;
the cases store it separately, because they give it different types (as case A(x string) and case B(x int, y int) do) or because the type has a method of that name, so Copy cannot tell which one is meant.

Minimal repro.

package main

sealed type Shape {
    case Circle(r float64, label string)
    case Sq(side float64, label string)
}

func main() {
    val s Shape = Sq(2.0, "s")
    Println(s.Copy(r = 9.0))
}

Error output.

error[GALA-E0092]: r is a field of Circle but not of Sq, and a Shape value may be any of its cases
  --> main.gala:10:20
   |
10 |     Println(s.Copy(r = 9.0))
   |                    ^ match the case and build it with the new r
   |
   = hint: match the case and build it with the new r; for example `case Circle(_, label) => Circle(r, label)`

Fix. Match the case and build it anew with the new field, keeping the other fields from the pattern. Every other case is left as it is. A field that every case has, such as label here, can still be set with Copy:

package main

sealed type Shape {
    case Circle(r float64, label string)
    case Sq(side float64, label string)
}

func grow(s Shape) Shape = s match {
    case Circle(_, label) => Circle(9.0, label)
    case _ => s
}

func main() {
    val s Shape = Sq(2.0, "s")
    Println(grow(s), grow(Circle(1.0, "c")), s.Copy(label = "t"))
}

On a constructor call the case is known, so Copy sets that case’s own fields: Sq(2.0, "s").Copy(side = 3.0).

Rationale. A sealed value holds the fields of one case. Copy used to check the name against the struct that stores every case’s fields, so it could set a field of another case: the copy printed as the original, yet was no longer equal to it.

Sealed Types — cases and their fields
All GALA error codes

GALA-E0092 — Copy names a field the sealed value cannot have

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