[Bug gdb/28946] New: Missing template paramater pack support

[Bug gdb/28946] New: Missing template paramater pack support

[jengelh at inai dot de]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

            Bug ID: 28946
           Summary: Missing template paramater pack support
           Product: gdb
           Version: 11.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: jengelh at inai dot de
  Target Milestone: ---

Input
=====
```
template<typename... T> void f(T&&...zzargs) {}
int main() { f(0); }
```

g++-11 -Wall -ggdb3 x.cpp

Observed output
===============
» gdb a.out
GNU gdb (GDB; openSUSE Tumbleweed) 11.1
...
(gdb) b main
(gdb) r
...
Breakpoint 1, main () at x.cpp:2
2       int main() { f(0); }
Missing separate debuginfos, use: zypper install
libgcc_s1-debuginfo-11.2.1+git1173-2.3.x86_64
libstdc++6-debuginfo-11.2.1+git1173-2.3.x86_64
(gdb) s
f<int> () at x.cpp:1
1       template<typename... T> void f(T&&...zzargs) {}
(gdb) info args
No arguments.
(gdb) p zzargs
No symbol "zzargs" in current context.


Expected output
===============

(gdb) info args
zzargs = {0}
(gdb) p zzargs
$1 = {0}

or something of the sort.

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[Bug gdb/28946] Missing template paramater pack support

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

Hannes Domani <ssbssa at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ssbssa at sourceware dot org

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[Bug gdb/28946] Missing template paramater pack support

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

--- Comment #1 from Hannes Domani <ssbssa at sourceware dot org> ---
Note that gcc currently doesn't write the name of the parameter pack in the
debug information:

 <2><95>: Abbrev Number: 5 (DW_TAG_GNU_formal_parameter_pack)
    <96>   DW_AT_decl_file   : 1
    <97>   DW_AT_decl_line   : 1
    <98>   DW_AT_decl_column : 35
 <3><99>: Abbrev Number: 6 (DW_TAG_formal_parameter)
    <9a>   DW_AT_type        : <0xaa>
    <9e>   DW_AT_location    : 2 byte block: 91 0       (DW_OP_fbreg: 0)
 <3><a1>: Abbrev Number: 0

Apparently this has been reported already some time ago:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70536

So even if gdb would make use of DW_TAG_GNU_formal_parameter_pack, it couldn't
show it as `zzargs` in the example.

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[Bug c++/28946] Missing template paramater pack support

[tromey at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

Tom Tromey <tromey at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-06
          Component|gdb                         |c++
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
                 CC|                            |tromey at sourceware dot org

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[Bug c++/28946] Missing template paramater pack support

[dblaikie at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

David Blaikie <dblaikie at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dblaikie at gmail dot com

--- Comment #2 from David Blaikie <dblaikie at gmail dot com> ---
+1 to this from a Clang developer.

Clang does name the template parameter pack.
```
0x0000039a:   DW_TAG_structure_type
                DW_AT_name      ("single_and_pack<int, int>")
                DW_AT_byte_size (1)
                DW_AT_decl_file
("/usr/local/google/home/blaikie/dev/scratch/test.cpp")
                DW_AT_decl_line (9)
                DW_AT_decl_column       (8)
                DW_AT_sibling   (0x000003b7)

0x000003a4:     DW_TAG_template_type_parameter
                  DW_AT_name    ("T1")
                  DW_AT_type    (0x00000094 "int")

0x000003ac:     DW_TAG_GNU_template_parameter_pack
                  DW_AT_name    ("Ts")

0x000003b0:       DW_TAG_template_type_parameter
                    DW_AT_type  (0x00000094 "int")

0x000003b5:       NULL

0x000003b6:     NULL
```

And there's an extra bonus reason from Clang's side:

I'm working on a mode for clang (called -gsimple-template-names) that will
produce DW_AT_names for templates without template parameters in some cases (so
in the above example "single_and_pack") and so working around this bug (missing
structural/API access to template parameters) by using the string name won't be
possible with debug info like this)

(total aside, though this is probably for a conversation on a gdb mailing list
- it does look like gdb does quite a bit of work that looks /a lot/ like it's
intending to support debug info like I described above... anyone happen to have
context on that?)

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[Bug c++/28946] Missing template paramater pack support

[ssbssa at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

--- Comment #3 from Hannes Domani <ssbssa at sourceware dot org> ---
Is related to (or a duplicate of) PR17272.

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[Bug c++/28946] Missing template paramater pack support

[dblaikie at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

--- Comment #4 from David Blaikie <dblaikie at gmail dot com> ---
(In reply to Hannes Domani from comment #3)
> Is related to (or a duplicate of) PR17272.

Yeah, dup seems fine

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[Bug c++/28946] Missing template paramater pack support

[tromey at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

Tom Tromey <tromey at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #5 from Tom Tromey <tromey at sourceware dot org> ---
Dup.

*** This bug has been marked as a duplicate of bug 17272 ***

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[Bug c++/28946] Missing template paramater pack support

[ed at catmur dot uk]

https://sourceware.org/bugzilla/show_bug.cgi?id=28946

Ed Catmur <ed at catmur dot uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ed at catmur dot uk

--- Comment #6 from Ed Catmur <ed at catmur dot uk> ---
Hi David, can you let me know whether clang would be able to make use of the
fix in #17272 (see attached patch, linked PR, linked ML post, whatever works
for you)? It would be useful to know whether someone would be able to make use
of it, even if gcc aren't interested in supporting
DW_TAG_GNU_template_parameter_pack.

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