Specialized template function with deleted “general” case fails to compile with g++ <=4.8.0 and clang++

问题

Compiling a project with an older version of g++ (4.8.0, MinGW) I found that this code fails to compile:

template<typename T>
void foo() = delete;

template<>
void foo<int>(){}

int main() {
    foo<int>();
    return 0;
}

It seems that g++ doesn't even try to look for explicit specializations if it sees that the base case is deleted.

mitalia@mitalia:~/scratch$ /opt/mingw32-dw2/bin/i686-w64-mingw32-g++ -std=c++11 buggy_deleted_template.cpp 
buggy_deleted_template.cpp: In function 'int main()':
buggy_deleted_template.cpp:8:14: error: use of deleted function 'void foo() [with T = int]'
     foo<int>();
              ^
buggy_deleted_template.cpp:5:6: error: declared here
 void foo<int>(){}
      ^
mitalia@mitalia:~/scratch$ /opt/mingw32-dw2/bin/i686-w64-mingw32-g++ --version 
i686-w64-mingw32-g++ (rubenvb-4.8.0) 4.8.0
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Instead, g++ 4.8.4 and 5.2 (on Linux) do not complain. Is this a bug in the older version of the compiler or a gray area in the standard?

---

Addendum

clang 3.4.1 too seems not to like it:

mitalia@mitalia:~/scratch$ clang++ -std=c++11 buggy_deleted_template.cpp                                                             
buggy_deleted_template.cpp:5:6: error: redefinition of 'foo'                                                                         
void foo<int>(){}
     ^
buggy_deleted_template.cpp:5:6: note: previous definition is here
buggy_deleted_template.cpp:8:5: error: no matching function for call to 'foo'
    foo<int>();
    ^~~~~~~~
buggy_deleted_template.cpp:2:6: note: candidate template ignored: substitution failure [with T = int]
void foo() = delete;
     ^
2 errors generated.
mitalia@mitalia:~/scratch$ clang++ --version
Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 3.4)
Target: x86_64-pc-linux-gnu
Thread model: posix

(and @Baum mit Augen in the comments reports that it still doesn't work in 3.7)

回答1:

I don't know if the following will be enlightening but I found defect report 941: Explicit specialization of deleted function template with status C++11 that states the following (Emphasis Mine):

According to 14.7.3 [temp.expl.spec] paragraph 1, only non-deleted function templates may be explicitly specialized. There doesn't appear to be a compelling need for this restriction, however, and it could be useful to forbid use of implicitly-instantiated specializations while still allowing use of explicitly-specialized versions.

Proposed resolution (February, 2010):

Change 14.7.3 [temp.expl.spec] paragraph 1 as follows:

An explicit specialization of any of the following:

non-deleted function template

class template

non-deleted member function of a class template

static data member of a class template

member class of a class template

member class template of a class or class template

non-deleted member function template of a class or class template

can be declared...

Now the current state of the draft standard N4527 is 14.7.3 Explicit specialization [temp.expl.spec]:

1 An explicit specialization of any of the following:

(1.1) — function template

(1.2) — class template

(1.3) — variable template

(1.4) — member function of a class template

(1.5) — static data member of a class template

(1.6) — member class of a class template

(1.7) — member enumeration of a class template

(1.8) — member class template of a class or class template

(1.9) — member function template of a class or class template

...

So I guess:

template<typename T>
void foo() = delete;

template<>
void foo<int>(){}

int main() {
    foo<int>();
    return 0;
}

Is C++11 standard compatible code and should be accepted.

来源：https://stackoverflow.com/questions/33256589/specialized-template-function-with-deleted-general-case-fails-to-compile-with