parser: empty match-arm body `Pat => {}` is a parse error

[hyperpolymath]

Summary

An empty match-arm body Pat => {} is a parse error, while a non-empty block body parses fine. Downstream svalinn-gateway source uses empty arms (e.g. None => {}).

Repro

Fails:

module m;
enum Opt { SomeV(Int), NoneV }
pub fn f(o: Opt) -> Int {
  match o {
    SomeV(v) => { return v; }
    NoneV => {}
  }
  return 0;
}

→ parse error: Syntax error at the } closing the match.

Parses (contrast — non-empty body):

    NoneV => { return 0; }

→ Type checking passed.

So the empty {} block body specifically is the limitation.

Question / direction

Is an empty arm body intended-unsupported, or a grammar gap? If empty arms should be legal (unit-valued), the parser should accept => {}; otherwise a clearer diagnostic than a bare "Syntax error" would help downstream authors.

Note

The other downstream-flagged construct, pub extern fn, was checked and is supported — both check and compile succeed — so no issue is filed for it.

Context

Surfaced characterising svalinn-gateway gaps after the constructor-link fix (#138 / #602, ac98c81). Confirmed at main b8ba479.