expectIf

fn expectIf(p: *Parse, comptime bodyParseFn: fn (p: *Parse) Error!Node.Index) !Node.Index {
    const if_token = p.assertToken(.keyword_if);
    _ = try p.expectToken(.l_paren);
    const condition = try p.expectExpr();
    _ = try p.expectToken(.r_paren);
    _ = try p.parsePtrPayload();

    const then_expr = try bodyParseFn(p);

    _ = p.eatToken(.keyword_else) orelse return try p.addNode(.{
        .tag = .if_simple,
        .main_token = if_token,
        .data = .{ .node_and_node = .{
            condition,
            then_expr,
        } },
    });
    _ = try p.parsePayload();
    const else_expr = try bodyParseFn(p);

    return try p.addNode(.{
        .tag = .@"if",
        .main_token = if_token,
        .data = .{ .node_and_extra = .{
            condition,
            try p.addExtra(Node.If{
                .then_expr = then_expr,
                .else_expr = else_expr,
            }),
        } },
    });
}