[Dwarf-discuss] Extensions for nested template type parameters

[Dave Blaikie]

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.

> 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.

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.

>
> 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.

- Dave