Thanks for bringing this up/working on these features.
I certainly do like the generalization to DW_TAG_{type,variable}_pack.
Are there other places where the type pack would show up, other than as a
template parameter? Might help inform the design a bit more.
I can't think of one, but I'd probably still vote for the
name/generalization so it matches better with variable_pack, and maybe has
some future uses we can't see yet.
On Wed, Sep 17, 2025 at 12:19 AM Michael Buch <[email protected]> wrote:
> Hi all,
>
> I have been working on some LLDB/debug-info issues surrounding parameter
> packs recently and stumbled upon following DWARFv6 proposal:
> https://dwarfstd.org/issues/250516.1.html
>
> I'm in favour of standardising parameter pack support in DWARF, but wanted
> to recommend a few changes to the above proposal to make sure we
> accommodate various use-cases not yet covered by it.
>
> At the time of the proposal there were two kinds of packs
> 1. template parameter packs
> 2. function parameter packs
>
> Since then two more kinds of packs have been added to the language:
> 3. lambda capture packs (C++20)
> 4. structured binding packs (C++26)
>
> The proposal currently describes (1) as DW_TAG_template_parameter_pack
> and (2) as DW_TAG_formal_parameter_pack
>
> However, it seems to me that neither of these are suitable to describe (3)
> and (4).
>
> # Lambda Capture Packs
>
> Here is an example of such pack:
> ```
> template <typename ... Args> auto f(Args&& ... args){ return [... args =
> std::forward<Args>(args)] { // use args }; }
> ```
>
> Here, `args` is actually a collection of init-capture declarations. In
> DWARF one might want to represent these as
> `DW_TAG_member`s on the anonymous lambda structure. However, neither
> DW_TAG_formal_parameter_pack
> nor DW_TAG_template_parameter_pack would allow such a representation.
>
> # Structured Binding Packs
>
> Here is an example of such pack:
> ```
>
> struct C { int x = 1, y = 2, z = 3; };
>
> void foo() {
> auto [d, ...e] = C();
> }
>
> ```
>
> Here, `e` is a collection of variable declarations. In DWARF one might
> want to represent these as `DW_TAG_variable`s local to the function `foo`.
> But, again, neither of the proposed new tags currently allows this.
>
> More discussion on that here:
> https://github.com/llvm/llvm-project/issues/152282
>
> # Proposed changes to the proposal
>
> A more general representation for packs would be having the notion of a
> "type pack" and a "variable pack". The "type pack" is a collection of
> types, which is what DW_TAG_template_paramaeter_pack encodes. A "variable
> pack" is a collection of variable declarations, which could be function
> parameters (DW_TAG_formal_parameter) or variables introduced with
> structured bindings (DW_TAG_variable) or even variables introduced via
> lambda capture packs (DW_TAG_member on the fake lambda structure).
>
>
> That way we could represent the DWARF forauto [a, ...e] like so:
>
> ```
>
> DW_TAG_variable
> DW_AT_name ("a")
>
> DW_TAG_variable_pack
> DW_AT_name ("e")
> DW_TAG_variable
> DW_AT_type
> DW_AT_name ("fake_var_1") << may or may not be needed. Debugger could
> synthesize this
> DW_TAG_variable
> DW_AT_type
> DW_AT_name ("fake_var_2") << may or may not be needed. Debugger could
> synthesize this
>
> ```
>
>
> Let me know if you have any questions/concerns with these suggestions. Happy
> to discuss!
>
>
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