[Dwarf-discuss] Extensions for nested template type parameters

[Ben Woodard]

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> 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
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