[Bug c++/82724] New: Larger than needed DWARF type declarations for explicitly instantiated class templates

["dblaikie at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82724

            Bug ID: 82724
           Summary: Larger than needed DWARF type declarations for
                    explicitly instantiated class templates
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dblaikie at gmail dot com
  Target Milestone: ---

When the vtable/key function debug info optimization kicks in, a declaration
(rather than a definition) of a type is produced:

$ cat foo.cpp
template <typename T> struct foo {
  T t;
  virtual ~foo() {}
};
extern template struct foo<int>;
foo<int> *f;
$ g++-6.3 foo.cpp -g -c && llvm-dwarfdump-tot foo.o | grep
"DW_TAG\|DW_AT_name\|DW_AT_declaration"
...
DW_TAG_structure_type
  DW_AT_name      ("foo<int>")
  DW_AT_declaration       (true)
  DW_TAG_template_type_parameter
    DW_AT_name    ("T")
...


But if the class template is only a declaration, then the DWARF is smaller -
the template_type_parameter is omitted (& thus anything that actual type
description and anything that type references is also omitted):

$ cat foo.cpp
template <typename T> struct foo;
foo<int> *f;
$ g++-6.3 foo.cpp -g -c && llvm-dwarfdump-tot foo.o | grep
"DW_TAG\|DW_AT_name\|DW_AT_declaration"
...
DW_TAG_structure_type
  DW_AT_name      ("foo<int>")
  DW_AT_declaration       (true)
...

This seems more compact and GDB knows how to cope with it, since it's been this
way forever - so it probably makes sense to make the declaration in the first
case look like the declaration in the second case so it's more compact.

LLVM/Clang already does this.