[Bug c++/47342] New: misleading diagnostic for member of undeclared class template partial specialization

[Bug c++/47342] New: misleading diagnostic for member of undeclared class template partial specialization

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47342

           Summary: misleading diagnostic for member of undeclared class
                    template partial specialization
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


g++ does not distinguish between invalid uses of an incomplete partial
specialization and an undeclared partial specialization:

template<class T>
  struct X : T
  {
    void f();
  };

template<class T>
  inline void
  X<T>::f()
  { }

template<typename T>
  struct Y
  { };

#ifdef CORRECT
template<class T>
  struct X< Y<T> > : Y<T>
  {
    void f();
  };
#endif

template<class T>
  inline void
  X< Y<T> >::f()    // xxx
  { }


$ g++ ps.cc
ps.cc:26: error: invalid use of incomplete type 'struct X<Y<T> >'
ps.cc:3: error: declaration of 'struct X<Y<T> >'

The diagnostic is rather unhelpful.

The problem is not that the type is incomplete, but that there is not even a
declaration of the class template partial specialization X<Y<T>> (which can be
fixed by defining CORRECT).  The second error referring to line 3 is wrong,
that's the declaration of the primary template, not the partial specialization.

The diagnostic would be almost correct (apart from the line number) if the
class template partial specialization had been declared but not defined, like
so
    template<class T> struct X< Y<T> >;
but that is not the case here.

This is a regression since 4.1 which correctly states the type is undefined,
not incomplete:
ps.cc:27: error: invalid use of undefined type `struct X<Y<T> >'
ps.cc:3: error: declaration of `struct X<Y<T> >'
ps.cc:27: error: template definition of non-template `void X<Y<T> >::f()'

EDG gives:
"ComeauTest.c", line 26: error: template argument list must match the parameter
list
    X< Y<T> >::f()    // xxx
    ^
This isn't very informative but is more accurate, referring to the wording in
[temp.class.spec.mfunc]p1:
The template argument list of a member of a class template partial
specialization shall match the template argument list of the class template
partial specialization.

Clang++ gives a more helpful diagnostic:
ps.cc:26:14: error: nested name specifier 'X<Y<T> >::' for declaration does not
      refer into a class, class template or class template partial
      specialization
  X< Y<T> >::f()    // xxx
  ~~~~~~~~~~~^
1 error generated.

Sun CC gives arguably the best diagnostic:
"ps.cc", line 27: Error: Could not find partial specialization matching
X<Y<T>>.
1 Error(s) detected.

(N.B. clang++ gives a worse diagnostic for the case where the class template
partial specialization has been declared but not defined and CC accepts it
without error)

[Bug c++/47342] misleading diagnostic for member of undeclared class template partial specialization

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47342

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-01-18 13:00:26 UTC ---
Oops, the g++ diagnostics I showed were from 4.4 not 4.6, but they're the same
except 4.6 adds column numbers

I've created PR 47343 for the incorrect line number in the case where the
partial specialization is declared but incomplete.

[Bug c++/47342] misleading diagnostic for member of undeclared class template partial specialization

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47342

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2017-09-26 00:00:00         |2021-5-12
           Severity|enhancement                 |normal

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I was about to report a new bug, but I think it's another case of this one:

template<typename T> struct S { };

template<typename T> struct X { };

template<typename T> struct S<X<T>>;  // line 5

void f(S<X<int>>* s)
{
  s->x;
}

This prints:

loc.C: In function 'void f(S<X<int> >*)':
loc.C:9:4: error: invalid use of incomplete type 'struct S<X<int> >'
    9 |   s->x;
      |    ^~
loc.C:1:29: note: declaration of 'struct S<X<int> >'
    1 | template<typename T> struct S { };
      |                             ^

The note shows the declaration of the primary template, which is not
incomplete.

The relevant declaration is the one on line 5.