[Dwarf-discuss] Extensions for nested template type parameters

[Cary Coutant]

Ben has submitted this as a proposal for the DWARF committee. This is Issue
260703.1:

https://dwarfstd.org/issues/260703.1.html

-cary


On Tue, Jun 9, 2026 at 9:00 AM Ben Woodard via Dwarf-discuss <
dwarf-discuss@lists.dwarfstd.org> wrote:

>
> On 6/8/26 4:21 PM, Dave Blaikie via Dwarf-discuss wrote:
>
> On Thu, Jun 4, 2026 at 6:00 PM Cary Coutant <ccoutant@gmail.com> <ccoutant@gmail.com> wrote:
>
> template<typename T> struct t1 {
>   T n1;
> };
>
> template<typename T> struct t2 {
>   T m1;
>   t1<T> m2;
> };
>
> int main()
> {
>   t2<int> v1;
>   return 0;
> }
>
> Based on the current specification, the type of m1 should reference
> a template parameter entry.  This is a bug in gcc which has already
> been described [1].  llvm/clang attempted to fix this in PR [2].
>
> Also for m2 we can see that there is no information that its type was
> originally described by a template type parameter of t2.  However,
> there is no specification addressing this specific case in the DWARF
> Standard.
>
> Like Ben, I'm not sure I see why this isn't possible given the current spec. The type attribute for v1 should point to a type DIE for t2<int>, which will have a template type parameter and two data members, m1 and m2. Briefly:
>
> L1: DW_TAG_structure_type (name: t2<int>)
> L2:   DW_TAG_template_type_parameter (name: T, type: ref to type int)
> L3:   DW_TAG_member (name: m1, type: ref to L2) [NOTE 1]
> L4:   DW_TAG_member (name: m2, type: ref to type t1<T>) [NOTE 2]
>
> NOTE 1: This is the bug you point out where gcc and clang don't do what the spec says, and put a ref to type int instead of to the template type parameter entry.
>
> NOTE 2: This is the part that may not be so obvious. We want a ref to a DIE that represents the type t1<T> rather than t1<int>. I think we can have that, but that new type entry will need to be a child of the t2<int> type. Otherwise, it won't be able to reference the template type parameter entry (technically, it *could*, but pointing from one type DIE into the interior of another is ugly, and could conceivably cause problems down the road).
>
> L1: DW_TAG_structure_type (name: t2<int>)
> L2:   DW_TAG_template_type_parameter (name: T, type: ref to type int)
> L3:   DW_TAG_member (name: m1, type: ref to L2)
> L4:   DW_TAG_member (name: m2, type: ref to L5)
> L5:   DW_TAG_structure_type (name: t1<T>)
> L6:     DW_TAG_template_type_parameter (name: T, type: ref to type int)
> L7:     DW_TAG_member (name: n1, type: ref to L6)
>
>
> Besides missing consumer support (e.g. GDB or LLDB), missing support for
> nested templates was one further reason why the llvm PR [2] was rejected
> by David Blaikie.  I discussed this topic with him since I am working on
> the enablement in GDB and submitted a patch series [3] to support template
> type resolution based on the current DWARF specification.  David Blaikie
> asked me to submit a DWARF issue for this, since we could not come up with
> a useful specification for the nested case (type of m2 in the example
> above).
>
> In the PR, David noted:
>
>
> That's basically my point, sorry, that v1 must be represented as "trait::type" (because it has to be canonical) and so if many uses, like this one, of a type in a template can't reference the DW_TAG_template_type_parameter, because it's non-canonical - I'm not sure there's a lot of value in making it work for the subset of cases where it is workable.
>
> I guess I don't understand the part about "it has to be canonical". David, can you explain why the approach I used above wouldn't work for the PR's example with trait::type?
>
> Consider some further use of t1<int> elsewhere (in some t3<int> that's
> similar to t2<int>, or independently, etc) - the consumer needs to
> know that t1<int> is the same type as t1<T> inside t1<int>.
> Also from a debug info size perspective, we wouldn't want to duplicate
> the type description of t2<int> with all its members, etc.
>
> That is specifically what I was thinking about in my two further scenarios.
>
> I'm going to argue that:
>
> t1<int> is not the same as the t1<T> declared inside of t2 even when t2 is
> instantiated with T being an int. They are certainly compatible types and
> the language rules probably allow something like:
>
> t1<int> foo;
>
> t2<int> bar;
> bar.m2=foo;
>
> but I would argue that they are different types from the perspective of
> DWARF.
>
> foo is a t1 instantiated with a T parameter referencing an int.
> bar.m2 is a t1 instantiated with a T (t1's T) parameter referencing t2's T
> parameter that then references an int.
>
> I believe that we NEED to convey that to consumers. C++ template
> metaprogramming is unbelievably hard to unscramble and trying to flatten
> everything down to its fundamental types is just going to make it harder.
>
> I think what bothers you is the combinatoric expansion that this can lead
> to within the type system and maybe the fact that DWARF types and the rules
> around them may need to differ slightly from the source language. However,
> the language and the DWARF actually are telling different stories. The
> compiler (with some help from the linker) wants to make sure that any
> program text may result from template expansion is not duplicated. This of
> course inclines producers toward flattening the template expansion. On the
> other hand, DWARF is there to connect the programmer's source view of the
> code they have written in the source language to the resulting binary after
> the producer has done its work. In this context, flattening the types in
> the template expansion misrepresents the programmers intent by presenting
> an intermediate step that the producer passed through when doing the
> template expansion.
>
> Remember if t1 had any methods, both the one from foo and the one from
> bar.m2 would point to the same program text. This isn't just about the size
> of the binary. It does not impact code generation. This is just debuginfo.
> It is about clearly showing the consumer the way that bar.m2 arrived at the
> instantiation t1<int> (where int is the
> unreferenced template type parameter) was by having t2 take a template
> parameter which was then passed to t1 as its template parameter.
>
> If we need something canonical (and if I understand what that means), could we also have a canonical instantiation of the type for t1<int> at the outermost scope, and add an attribute to the entry for t1<T> at L5 that points to that canonical instantiation — maybe call it DW_AT_canonical_type?
>
> Possibly, though it gets verbose (we would want tnhe t1<T> at L5 to be
> only a declaration, not a definition, for instance - so it doesn't
> contain a duplicate description of all of t1<int>'s members). In the
> above case, we could emit t1<T> as a typedef, or at least handle it
> similarly to a typedef.
>
> And there are more complicated situations, like this:
>
> template<typename T1>
> struct t1 {
>   typedef U1 = T1;
> };
> template<typename T2>
> struct t2 {
>   typedef U2 = t1<T2>::U1;
> };
> int main() {
>   t2<int>::U2 x;
> }
>
> Following the above direction you're suggesting Cary, I suppose we
> could still emit a declaration for t1<T2> inside t2<int>, and have
> that declaration contain the U1 typedef referring to the T1 parameter
> (with a value of T2) - though it does feel a bit awkward/verbose to
> me. In template heavy code this may produce a lot of new declarations
> to describe these template parameter mappings.
>
> See my arguments above.
>
> I think there's probably some worse cases of template parameter use
> I'm not thinking of... but haven't come up with them yet.
>
> If you want to see truly evil example of template metaprogramming, the
> likes of which will make your eyes bug and leave you with nightmares about
> being lost in a maze of twisty little passages which all look about the
> same, may I suggest Raja https://github.com/llnl/raja or its slightly
> better mannered cousin Kokkos https://github.com/kokkos/kokkos
>
> There's also the concern that the compilers currently don't generate what the DWARF spec says they should, and if they did, the debuggers wouldn't be able to handle it. And if David's point above is right that we can represent the simple case, but not the more complex ones, is it worth doing it even for the simple cases? If the spec is calling for something that can't be done in the general case, should we remove it from the spec?
>
> That'd be the direction I'd lean. I'd at least be inclined to leave it
> out/not say one way or the other - if a producer and consumer can
> agree to do this sort of thing in some cases and they're OK with it
> not being a complete solution, I wouldn't be so firm as to say they
> shouldn't be allowed to - just that in my capacity as an LLVM/clang
> debug info maintainer, I don't think it's the right direction
> there/anywhere I'd have ownership over, at my current understanding of
> the situation.
>
> I worry about timeline here. I think with DWARF6 we at least have
> something that at least _could_ work. There will be a big push in both
> producers and consumers to support DWARF6. I think that these things (which
> are arguably bugs in current toolchain's DWARF5 support) should be part of
> that enablement. I'll file the bugs and track them, My involvement with
> DWARF6 probably is going to continue through to the implementation in the
> toolchains.
>
> I believe that what you are really hoping for is a new design for how to
> support template instantiation in DWARF. With the rise of STL and generic
> programming and now with template metaprogramming, template instantiation
> is a much bigger part of C++ than it was when the current template
> instantiation support was added to DWARF and we are starting to see some of
> the problems with that original design. This problem being one of the
> problems. Another one is we have a huge amount of highly similar almost
> duplicated debuginfo. However, I feel like taking on that design challenge
> of how to more compactly represent the vastly increased use of templates in
> C++ is more of A DWARF7 project than a DWARF6 project.
>
> Unlike the DWARF for GPUs project which we are refining and hope to fully
> land in DWARF6, where we had a fairly complete design and implementation
> based on DWARF5, I have not even heard of the beginnings of discussions
> about a new model design for template instantiation yet. This is why I
> think it is best to push that off to DWARF7. I think we should add an
> example or two to D.11 about how this should work based upon what Cary and
> I have suggested for DWARF6, Then as part of the overall DWARF6 enablement
> in the toolchains, we get both the producers and consumers to fall in line
> with that standard.
>
> Then if possible, we come up with a better design for DWARF7. I believe
> that such a design may have at its core, a reconsideration of the policy of
> DWARF toward templates: "*DWARF does not represent the generic template
> definition, but does represent each instantiation.*" (ref section 2.22)
> However, starting with a reconsideration of that premise and marching all
> the way through to a workable design which handles all the scenarios that
> come up in modern C++ and in the even more modern current C++ standards
> where template meta programming are a much bigger factor is going to be
> quite a journey.
>
> -ben
>
> - Dave
>
> --
> Dwarf-discuss mailing list
> Dwarf-discuss@lists.dwarfstd.org
> https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.dwarfstd.org/pipermail/dwarf-discuss/attachments/20260703/837a585e/attachment-0001.htm>