[Bug c++/59254] New: confusing diagnostic for undefined template shadowed by declaration in inline namespace

[Bug c++/59254] New: confusing diagnostic for undefined template shadowed by declaration in inline namespace

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59254

            Bug ID: 59254
           Summary: confusing diagnostic for undefined template shadowed
                    by declaration in inline namespace
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

namespace outer {
  inline namespace inner {
#ifndef OK
    template<typename T> struct A;
#endif
  }

  template<typename T> struct A;

  template class A<int>;
}

This gives a confusing diagnostic:

$ g++ -std=gnu++11 n.cc -c
n.cc:10:18: error: ‘A’ is not a class template
   template class A<int>;
                  ^
n.cc:10:18: error: template argument required for ‘class A’


That's particularly confusing because A is a class template, and so is inner::A
(although if it's just a class type the error is the same) and because a
template argument is given.


Without the previous definition in the inline namespace we get the expected
explanation:

$ g++ -std=gnu++11 n.cc -c -DOK
n.cc:10:18: error: explicit instantiation of ‘class outer::A<int>’ before
definition of template
   template class A<int>;
                  ^
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Subject: [Bug target/59216] [4.9 Regression] ARM negdi*extendsidi regression
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY216

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-11-22
     Ever confirmed|0                           |1

--- Comment #3 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
Confirmed, patch forthcoming.

[Bug c++/59254] confusing diagnostic for undefined template shadowed by declaration in inline namespace

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59254

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-03-23
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Confirmed on trunk.

The difference between 'struct' and 'class' in my original example is not
relevant:

namespace outer {
  inline namespace inner {
    template<typename T> struct A;
  }

  template<typename T> struct A;

  template struct A<int>;
}

[Bug c++/59254] confusing diagnostic for undefined template shadowed by declaration in inline namespace

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59254

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-03-23 00:00:00         |2020-7-29

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If I add a declaration of an explicit specialization A<long> we get another
unhelpful error:

namespace outer {
  inline namespace inner {
    template<typename T> struct A;
  }

  template<typename T> struct A;

  template struct A<int>;

  template<> struct A<long>;
}

n.cc:8:19: error: 'A' is not a class template
    8 |   template struct A<int>;
      |                   ^
n.cc:8:19: error: template argument required for 'struct A'
n.cc:10:21: error: 'A' is not a class template
   10 |   template<> struct A<long>;
      |                     ^
n.cc:10:21: error: template specifiers not specified in declaration of
'template<class T> struct outer::A'


I have no idea what "template specifiers not specified in declaration" means.

Clang is more clear about the problem (the class template is declared twice and
so causes ambiguities):


n.cc:8:19: error: reference to 'A' is ambiguous
  template struct A<int>;
                  ^
n.cc:6:31: note: candidate found by name lookup is 'outer::A'
  template<typename T> struct A;
                              ^
n.cc:3:33: note: candidate found by name lookup is 'outer::inner::A'
    template<typename T> struct A;
                                ^
n.cc:8:19: error: explicit instantiation of undefined template 'outer::A<int>'
  template struct A<int>;
                  ^
n.cc:6:31: note: template is declared here
  template<typename T> struct A;
                              ^
n.cc:10:21: error: reference to 'A' is ambiguous
  template<> struct A<long>;
                    ^
n.cc:6:31: note: candidate found by name lookup is 'outer::A'
  template<typename T> struct A;
                              ^
n.cc:3:33: note: candidate found by name lookup is 'outer::inner::A'
    template<typename T> struct A;
                                ^

EDG does well too:

"n.cc", line 8: error: "outer::A" is ambiguous
    template struct A<int>;
                    ^

"n.cc", line 8: error: invalid explicit instantiation declaration
    template struct A<int>;
             ^

"n.cc", line 10: error: "outer::A" is ambiguous
    template<> struct A<long>;
                      ^

3 errors detected in the compilation of "n.cc".