This may be dup case of PR102033, PR102034 and very probably not related with PR102042 of which I just changed the function parameter of "g" to const array instead of const reference of array. So, it can be another test case. However, clang(https://www.godbolt.org/z/fxzoWK4G8) passes, MSVC++(https://www.godbolt.org/z/c7Pfbr6b9) fails like GCC. In this sense, I believe this case might be different from all above cases because MSVC++ does not fail. consider: #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>)){} 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(const fPtr<1,int>[1])>::value, "fun"); // #2 template<> void g<1,int>(const fPtr<1,int>[1]){} GCC says: error: template-id 'g<1, int>' for 'void g(void (* const*)(const int*))' does not match any template declaration 21 | void g<1,int>(const fPtr<1,int>[1]){} | ^~~~~~~~ fun-array-value.cpp:12:6: note: candidate is: 'template<unsigned int N, class T> void g(decltype (af<N, T>))' 12 | void g(const decltype(af<N,T>)){} | ^
This looks like another case where arrays are not decaying correctly; either too soon or too late.
The root cause of this issue maybe similar to those PR102033, PR102034, PR102042 etc., however, this is still a distinctive case because of its nature. It is not a "typename" indicating its a dependent-type, rather using "decltype". So, in this sense, to tackle this issue may need a different approach. Currently I haven't found a similar function to resolve "decltype" type similar to "resolve_typename_type". If anybody knows one, please let me know.
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>: https://gcc.gnu.org/g:79802c5dcc043a515f429bb2bec7573b8537c32a commit r12-4453-g79802c5dcc043a515f429bb2bec7573b8537c32a Author: Jason Merrill <jason@redhat.com> Date: Tue Sep 28 10:02:04 2021 -0400 c++: array cv-quals and template specialization [PR101402] PRs 101402, 102033, etc. demonstrated that the fix for PR92010 wasn't handling all cases of the CWG1001/1322 issue with parameter type qual stripping and arrays with templates. The problem turned out to be in determine_specialization, which did an extra substitution without the 92010 fix and then complained that the result didn't match. But just removing that wrong/redundant code meant that we were accepting specializations with different numbers of parameters, because the code in fn_type_unification that compares types in this case wasn't checking for length mismatch. After fixing that, I realized that fn_type_unification couldn't tell the difference between variadic and non-variadic function types, because the args array doesn't include the terminal void we use to indicate non-variadic function type. So I added it, and made the necessary adjustments. Thanks to qingzhe "nick" huang <nickhuang99@hotmail.com> for the patch that led me to dig more into this, and the extensive testcases. PR c++/51851 PR c++/101402 PR c++/102033 PR c++/102034 PR c++/102039 PR c++/102044 gcc/cp/ChangeLog: * pt.c (determine_specialization): Remove redundant code. (fn_type_unification): Check for mismatched length. (type_unification_real): Ignore terminal void. (get_bindings): Don't stop at void_list_node. * class.c (resolve_address_of_overloaded_function): Likewise. gcc/testsuite/ChangeLog: * g++.dg/template/fnspec2.C: New test. * g++.dg/template/parm-cv1.C: New test. * g++.dg/template/parm-cv2.C: New test. * g++.dg/template/parm-cv3.C: New test.
fixed by patch under PR101402