[Bug c++/64063] New: Incorrect "ambiguous template specialization" error

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64063

            Bug ID: 64063
           Summary: Incorrect "ambiguous template specialization" error
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

G++ rejects this explicit instantiation definition as ambiguous:

template<typename T>
struct S
{
  void foo(int) { }
  template<typename U> void foo(U) { }
};

template void S<char>::foo(int);  // XXX


EDG thinks it's OK, clang crashes, and G++ says:


inst.cc:8:15: error: ambiguous template specialization ‘foo<>’ for ‘void
S<char>::foo(int)’
 template void S<char>::foo(int);  // XXX
               ^
inst.cc:4:8: note: candidates are: void S<T>::foo(int) [with T = char]
   void foo(int) { }
        ^
inst.cc:5:29: note:                 template<class U> void S<T>::foo(U) [with U
= U; T = char]
   template<typename U> void foo(U) { }
                             ^


I think EDG is correct and the diagnostic is wrong, because I'm not referring
to any foo<> but rather foo.


Changing the instantiation to be unambiguously a template produces almost the
same error (with foo<> replaced by foo<int>) and that definitely isn't
ambiguous because one candidate isn't even a function template:

template<typename T>
struct S
{
  void foo(int) { }
  template<typename U> void foo(U) { }
};

template void S<char>::foo<int>(int);  // XXX


inst.cc:8:15: error: ambiguous template specialization ‘foo<int>’ for ‘void
S<char>::foo(int)’
 template void S<char>::foo<int>(int);  // XXX
               ^
inst.cc:4:8: note: candidates are: void S<T>::foo(int) [with T = char]
   void foo(int) { }
        ^
inst.cc:5:29: note:                 template<class U> void S<T>::foo(U) [with U
= U; T = char]
   template<typename U> void foo(U) { }
                             ^
>From gcc-bugs-return-468449-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Nov 24 23:38:12 2014
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From: "ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug ada/64057] Overlapping memcpy generated for array assignment
Date: Mon, 24 Nov 2014 23:38:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd057

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2014-11-24
                 CC|                            |ebotcazou at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Let me know if you need more info

We need a testcase that fails with GCC 4.8 or GCC 4.9 since xsinfo is built by
the base compiler.