GALA-E0018.md

GALA-E0018 — Sealed variant type parameter cannot be inferred

When it fires. A zero-arg sealed-variant constructor is used in a position where the parent sealed type's parameter cannot be pinned. The transpiler walks three signals before giving up:

1. The enclosing match subject's type (e.g. cmd match { case NoCmd() => ... }).
2. The enclosing function's declared return type.
3. A local val/var annotation supplying an expected type.

If none of those resolve the parameter, generated Go would have to emit a literal Variant{} whose type parameter Go cannot deduce — producing an obscure cannot infer T error far from the GALA source. This code surfaces the failure at the GALA call site instead.

Minimal repro.

package main

sealed type Box[T] = case Empty()

func main(): Unit = {
    val x = Empty()  // No annotation, no enclosing type — parameter T is unbound
    Println(x)
}

Error output.

[SemanticError GALA-E0018] line 5:13 cannot infer type parameter for sealed variant constructor "Empty()" of "Box" (hint: annotate the variable (e.g. `val x: Box[Int] = Empty()`) or pass type args explicitly (`Empty[Int]()`))

Fix. Pick the form that documents intent best:

val x: Box[Int] = Empty()   // declarative; reads as a binding, not a call
val x = Empty[Int]()         // explicit instantiation; useful when the type param is the operative information

Or, when the constructor is an arm of a match against a value of the parent type, the inference signal is already present and no annotation is needed.

Rationale. GALA's downward inference relies on a single expected-type signal flowing from the enclosing context. When that signal is absent, the transpiler has historically fallen through to an untyped composite literal and let Go deduce — which fails confusingly. Failing fast, with a hint that names the three resolving signals, points the user at the cheapest fix.

Scope. Only zero-arg sealed-variant constructors of generic sealed types. Variants of non-generic sealed types and constructors that take arguments contributing to inference are unaffected.