positional_field_in_object_pattern

Object patterns can only use named fields.

The analyzer produces this diagnostic when an object pattern contains a field without specifying the getter name. Object pattern fields match against values that the object's getters return. Without a getter name specified, the pattern field can't access a value to attempt to match against.

The following code produces this diagnostic because the object pattern String(1) doesn't specify which getter of String to access and compare with the value 1:

void f(Object o) {
  if (o case String(1)) {}
}

Add the getter name to access the value, followed by a colon before the pattern to match against:

void f(Object o) {
  if (o case String(length: 1)) {}
}