Failure on partial template specialization with gcc 9.3, was working before

Question

I checked This stackoverflow post, but I still cannot understand.

I have this code (yes I know <functional> achieves the same thing) which worked (if memory serves me right, it was gcc6) and now fails with gcc9.3.0

Given these class definitions

template <typename D>
class Callback{
public:
  virtual void operator()(D d)=0;
  virtual ~Callback(){}
};

template <>
class Callback <void>{
public:
  virtual void operator()()=0;
  virtual ~Callback(){}
};

and these definitions

template <typename D,typename P>
struct ParamFunctionType{
  typedef void (*value)(D,P);
};

template <typename P>
struct ParamFunctionType<void,P>{
  typedef void (*value)(P);
};

I have these subclasses

template <typename D, typename P, typename ParamFunctionType<D,P>::value f>
class FunctionParamCallback:public Callback<D>
{
    P p;

    FunctionParamCallback(const P& pp) : p(pp) {}
       
    void operator()(D d) { (*f)(d,p); }
};

template <typename P, void (*f)(P)>
class FunctionParamCallback<void, P, f> : public Callback<void>
{
    P p;

    FunctionParamCallback(const P& pp) : p(pp) {}
    void operator()() { (*f)(p); }
};

Now the second definition seems a specialization to me (for D set to void), but I get the following error

 error: partial specialization ‘class helium::scb::FunctionParamCallback<void, P, f>’ is not more specialized than [-fpermissive]
  297 |     template <typename P,void (*f)(P)> class FunctionParamCallback<void,P,f>:public Callback<void>{
      |                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../helium/signal/callback.h:284:85: note: primary template ‘template<class D, class P, typename helium::scb::ParamFunctionType<D, P>::value f> class helium::scb::FunctionParamCallback’
  284 |     template <typename D,typename P,typename ParamFunctionType<D,P>::value f> class FunctionParamCallback:public Callback<D>{
  |

What am I mistaking?

(a MVCE is provided by Jarod42 here).

I would say gcc bug, [Demo](https://godbolt.org/z/vqYEEz) works in gcc with C++17 or more, and works with clang/msvc. — Jarod42, Jan 16 '21 at 10:17
@Jarod42 pls correct if I am wrong with my answer below. Thanks :) — NutCracker, Jan 16 '21 at 10:30
@Jarod42 thank you for the MCVE! I will add a comment to the original post. — Fabio Dalla Libera, Jan 17 '21 at 04:02

score 2 · Accepted Answer · answered Jan 16 '21 at 10:27

2

This is a GCC bug filed here if I am not mistaken. It is introduced in GCC 7.1.

answered Jan 16 '21 at 10:27

NutCracker

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1

thank you, Comment 4 provides the reason. – Fabio Dalla Libera Jan 17 '21 at 04:05

score 1 · Answer 2 · edited Jan 17 '21 at 08:30

Following the reasoning of comment 4 of this bug filing provided by @NutCracker I modified the code as follows

template <typename P, typename ParamFunctionType<void, P>::value f>
class FunctionParamCallback<void, P, f> : public Callback<void>
{ /*..*/};

This prevents the possibly inconsistent resolution for the type of f, and compiles in gcc9.3 without need to relax the checks through compilation options.

Failure on partial template specialization with gcc 9.3, was working before

2 Answers2