039 branch payload requires binding

// Test: Branch with payload requires binding or explicit discard
//
// When a branch has a payload (e.g., `result { value: i32 }`), the branch
// handler MUST either bind it or explicitly discard with `_`:
//
//   | result r |>    // LEGAL - binds payload to 'r'
//   | result _ |>    // LEGAL - explicitly discards payload
//   | result |>      // ILLEGAL - payload exists but no binding
//
// This test verifies the compiler catches the illegal case.

const std = @import("std");

~event compute { x: i32 }
| result { value: i32 }

~proc compute {
    return .{ .result = .{ .value = x * 2 } };
}

// BUG: This should be a compile error!
// The `result` branch has payload `{ value: i32 }` but no binding.
// Correct syntax would be: `| result _ |>` or `| result r |>` (if `r` is used)
~compute(x: 42)
| result |> std.io:println(text: "done")