[Bug c++/102042] New: specialization after instantiation error possibly rooted from mis-calculating template function signature of parameter of array of template function

[Bug c++/102042] New: specialization after instantiation error possibly rooted from mis-calculating template function signature of parameter of array of template function

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102042

            Bug ID: 102042
           Summary: specialization after instantiation error possibly
                    rooted from mis-calculating template function
                    signature of parameter of array of template function
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickhuang99 at hotmail dot com
  Target Milestone: ---

I suspect this is from similar root cause of PR101402, PR102033, PR102034 etc.
when incorrectly dropping top-level cv-qualifier during calculating template
function declarator of which parameter is an array of template function
pointer.
Clearly from "#2" of static assert, GCC already gets correct specialization
"g<1,int>" signature, still the last statement of specialization declaration
causes error with very strange error message. 

clang(https://www.godbolt.org/z/5eM5b4GjG) accepts.
MSVC++(https://www.godbolt.org/z/3x9dvsfTh) stumbles like GCC, but a better
clear clue.


#include<type_traits>
template<unsigned int N, class T>
void f(const T[N]){}

template<unsigned int N, class T>
using fPtr=decltype(f<N,T>)*;

template<unsigned int N, class T>
fPtr<N,T> af[N]={&f<N,T>}; // even without initialization,error still is

template<unsigned int N, class T>
void g(const decltype(af<N,T>)&){}

static_assert(std::is_same<decltype(af<1,int>),
 fPtr<1,int>[1] >::value, "af is correct"); // #1

static_assert(std::is_same<decltype(g<1,int>),
 void(fPtr<1,int>const(&)[1])>::value, "fun"); // #2

template<>
void g<1,int>(fPtr<1,int>const(&)[1]){}



GCC gives very misleading error message:

error: specialization of 'void g(decltype (af<N, T>)&) [with unsigned int N =
1; T = int; decltype (af<N, T>) = void (* [1])(const int*)]' after
instantiation
   21 | void g<1,int>(fPtr<1,int>const(&)[1]){}
      |                                     ^

[Bug c++/102042] specialization after instantiation error possibly rooted from mis-calculating template function signature of parameter of array of template function

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102042

--- Comment #1 from qingzhe huang <nickhuang99 at hotmail dot com> ---
Slightly change the prototype of "g" from reference to pointer, you have exact
same error. (please note #0 changes to pointer type)


#include<type_traits>
template<unsigned int N, class T>
void f(const T[N]){}

template<unsigned int N, class T>
using fPtr=decltype(f<N,T>)*;

template<unsigned int N, class T>
fPtr<N,T> af[N]={&f<N,T>};

template<unsigned int N, class T>
void g(const decltype(af<N,T>)*){} // #0

static_assert(std::is_same<decltype(g<1,int>),
 void(const fPtr<1,int>(*)[1])>::value, "fun"); // #2

template<>
void g<1,int>(const fPtr<1,int>(*)[1]){}



error: specialization of 'void g(decltype (af<N, T>)*) [with unsigned int N =
1; T = int; decltype (af<N, T>) = void (* [1])(const int*)]' after
instantiation
   24 | void g<1,int>(const fPtr<1,int>(*)[1]){}
      |

[Bug c++/102042] specialization after instantiation error possibly rooted from mis-calculating template function signature of parameter of array of template function

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102042

--- Comment #2 from qingzhe huang <nickhuang99 at hotmail dot com> ---
Just tested with my fix, it can be fixed with my suggested patch in PR102033.

[Bug c++/102042] specialization after instantiation error possibly rooted from mis-calculating template function signature of parameter of array of template function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102042

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another dup then. Your patch should include this as a testcase too (and should
be submitted to the mailing list).

*** This bug has been marked as a duplicate of bug 102033 ***