A slot kind names one child position, so the generator emits a single
typed Slots.X constant for it. A kind used with two different child types
had to widen that constant to AstNode, and the typed GetChildren<T>
accessor then cast the live AstNodeCollection<Concrete> to
AstNodeCollection<AstNode> -- an InvalidCastException waiting for the
first caller (latent today, but the model permitted it).
Make the loose state unrepresentable instead of guarding it at runtime:
DSTG001 errors when a [Slot] kind is declared with more than one child
type. The six kinds that only coincidentally shared a name (Body, True,
False, Initializer, SwitchSection, Variable) are split into precisely
typed kinds; a genuinely either/or position (a lambda body) stays one
declared type, AstNode. Every Slots.X now carries its real element type,
so the GetChildren cast can no longer fail.
Output is unchanged: the Pretty suite stays byte-identical with
CheckInvariant green in DEBUG.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Siegfried Pammer
e264e926bb