[Bug c++/86747] [8/9 Regression] rejects-valid with redundant friend declaration

[Francois-R.Boyer at PolyMtl dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86747

François-R Boyer  changed:

   What|Removed |Added

 CC||Francois-R.Boyer at PolyMtl 
dot ca

--- Comment #3 from François-R Boyer  ---
I'm not sure if this bug is related or if I should post it as a separate bug
report.  I have the same error messages, with a friend class, but in my case it
is not redundant, and happens in a much more complicated case.  Here is the
code:

template  struct A {
template  class> struct B {
template  class> friend struct B;
};
};

template  struct C { };

A::B b;


The above compiles on latest MSVC, clang, and most other compilers on godbolt,
including g++ 7.3. The error with g++ 8.1, 8.2 and 9.0.0 20181007 on godbolt
is:

: In instantiation of 'struct A::B':
:9:14:   required from here
:2:32: error: template parameter 'template template
class'
2 | template  class> struct B {
  |^
:3:57: error: redeclared here as 'template
class'
3 | template  class> friend struct B;
  | ^

Note that if struct A is not a template, of if struct B has a simple type
parameter instead of a template template, it compiles.

[Bug c++/87571] New: [8/9 Regression] ICE in friend_accessible_p, accessing protected member of template friend inside template class

[Francois-R.Boyer at PolyMtl dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87571

Bug ID: 87571
   Summary: [8/9 Regression] ICE in friend_accessible_p, accessing
protected member of template friend inside template
class
   Product: gcc
   Version: 9.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: Francois-R.Boyer at PolyMtl dot ca
  Target Milestone: ---

The following code fails to compile with g++ 8.1, 8.2 and 9.0.0 20181007 on
godbolt, but compiles with g++ 7.3, clang, MSVC, etc.:

template  struct A {
  template  struct B {
template  friend class B;
  protected:
int protected_member_;
  public:
template  int method(const B& other) const {
  return other.protected_member_;
}
  };
};

int main() {
  A::B a;
  A::B b;
  a.method(b);
}

The error message is:
: In instantiation of 'int A<  >::B<
 >::method(const A<  >::B&)
const [with T = char;  = int; 
= int]':
:16:13:   required from here
:8:20: internal compiler error: in friend_accessible_p, at
cp/search.c:719
8 |   return other.protected_member_;
  |  ~~^


Note that it compiles if we change any of these:
- the member is private instead of protected,
- struct A is not a template, or
- we explicitly 'friend class B;' instead or in addition of the template
friend.