GALA-E0073 — Invalid named sub-pattern

When it fires. A named sub-pattern Field = p matches p against the field of that name (Named Sub-Patterns). The error fires when one is malformed:

the field does not exist, as Depth below;
a field is matched twice, by name twice or by position and by name;
a positional sub-pattern follows a named one;
there are more positional sub-patterns than fields;
what the pattern matches has no fields to name: an extractor with its own Unapply, or a sequence such as Array, whose pattern matches elements. (An opaque-type pattern takes one sub-pattern and reports a named one itself.)

Minimal repro.

package main

sealed type Shape {
    case Circle(Radius int)
    case Rect(Width int, Height int)
}

func height(s Shape) int = s match {
    case Circle(Radius = r) => 2 * r
    case Rect(Depth = d)    => d
}

func main() {
    Println(height(Rect(1, 2)))
}

Error output.

error[GALA-E0073]: 'Rect' has no field 'Depth'
  --> main.gala:10:15
   |
10 |     case Rect(Depth = d)    => d
   |               ^^^^^ its fields are Width, Height
   |
   = hint: its fields are Width, Height

Fix. Name a field the variant or struct declares, once, after any positional sub-patterns. Fields left out match anything:

package main

sealed type Shape {
    case Circle(Radius int)
    case Rect(Width int, Height int)
}

func height(s Shape) int = s match {
    case Circle(Radius = r) => 2 * r
    case Rect(Height = h)   => h
}

func main() {
    Println(height(Rect(1, 2)))
}

For an extractor with its own Unapply, write the sub-patterns by position: its result has no field names.

Rationale. Named sub-patterns mirror named arguments at construction, so Rect(Height = h) reads the field it names. A name that matches no field, or a field matched twice, has no meaning to fall back on, so it is an error rather than a positional guess.