`textDocument/selectionRange` panics on invalid variadic function declaration

[hongtaihu]

Zig Version

0.16.0

ZLS Version

4944862

Client / Code Editor / Extensions

plain zls with debug

Steps to Reproduce and Observed Behavior

1

reduced from https://github.com/ziglang/zig/blob/master/test/cases/compile_errors/invalid_variadic_function.zig

  fn foo(...) void {
  }

2

send a textDocument/selectionRange request

3

stacktrace:

info  ( main ): Starting ZLS      0.16.0 @ '<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/zig-out/bin/zls'
info  ( main ): Log File:         <HOME>/.cache/zls/zls.log (debug)
debug (server): Offset Encoding:  'utf-16'
info  (server): added Workspace Folder: file://<REPO_ROOT>/harness/cases/CASE-20260524-005/workspace (2937 files)
thread 28698 panic: attempt to use null value
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/ast.zig:869:40: 0x19ef4f5 in paramFirstToken (zls.zig)
        tree.firstToken(param.type_expr.?);
                                       ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/ast.zig:877:40: 0x19ef22f in paramLoc (zls.zig)
    const first_token = paramFirstToken(tree, param, include_doc_comment);
                                       ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/features/selection_range.zig:57:55: 0x1a7acd8 in generateSelectionRanges (zls.zig)
                        const param_loc = ast.paramLoc(tree, param, true);
                                                      ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/Server.zig:1581:55: 0x1a79ea3 in selectionRangeHandler (zls.zig)
    return try selection_range.generateSelectionRanges(arena, handle, request.positions, server.offset_encoding);
                                                      ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/Server.zig:1825:76: 0x1a79870 in sendRequestSync__anon_38682 (zls.zig)
        .@"textDocument/selectionRange" => try server.selectionRangeHandler(arena, params),
                                                                           ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/Server.zig:1874:58: 0x1343427 in processMessage (zls.zig)
                const result = try server.sendRequestSync(arena, @tagName(method), params);
                                                         ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zls-0.16.0/src/Server.zig:1891:30: 0x133045e in processMessageReportError (zls.zig)
    if (server.processMessage(arena_allocator.allocator(), message)) |json_message| {
                             ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/Io.zig:1245:17: 0x132fe32 in start (std.zig)
                _ = @as(Cancelable!void, @call(.auto, function, args_casted.*)) catch {};
                ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/Io/Threaded.zig:552:22: 0x1303865 in start (std.zig)
            task.func(task.contextPointer());
                     ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/Io/Threaded.zig:1797:29: 0x1301a64 in worker (std.zig)
            runnable.startFn(runnable, &thread, t);
                            ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/Thread.zig:422:13: 0x1301745 in callFn__anon_27002 (std.zig)
            @call(.auto, f, args);
            ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/Thread.zig:1431:30: 0x1301500 in entryFn (std.zig)
                return callFn(f, self.fn_args);
                             ^
<REPO_ROOT>/tmp/zls-bootstrap-0.16.0/zig-x86_64-linux-0.16.0/lib/std/os/linux/x86_64.zig:105:5: 0x1301665 in clone (std.zig)
    asm volatile (

Expected Behavior

It should not crash.

Attach ZLS log output

No response