[llvm-bugs] [Bug 49684] New: Bug with const and partial template spepcialization in C++17

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49684

            Bug ID: 49684
           Summary: Bug with const and partial template spepcialization in
                    C++17
           Product: clang
           Version: 11.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eldlistmailingz at tropicsoft.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

The code:

#include <iostream>
#include <cstddef>
#include <type_traits>

template <class T,T (*g)(),bool (*s)(T),class NP = std::nullptr_t,class dummy =
void>  struct pfun;

template <class T,T (*g)(),bool (*s)(T)>
struct pfun<T,g,s,std::nullptr_t,typename std::enable_if<g != nullptr &&
(std::is_const<T>::value || s == nullptr)>::type>
  {
  pfun() {std::cout << "class pfun<T,g,s,std::nullptr_t,typename
std::enable_if<g != nullptr && (std::is_const<T>::value || s ==
nullptr)>::type>\n";} 
  };

char const gl_r_const_ch('c');
char const gl_fun_r_read_const_ch()  { return gl_r_const_ch; }
pfun<char const,&gl_fun_r_read_const_ch,nullptr> pf_read_const_ch;

// char gl_r_ch('c');
// char gl_fun_r_read_ch()  { return gl_r_ch; }
// pfun<char,&gl_fun_r_read_ch,nullptr> pf_read_ch;

int main() { return 0; }

Compiled with clang 11.0 the result is:

"test_pmf.cpp:15:50: error: implicit instantiation of undefined template
'pfun<const char, &gl_fun_r_read_const_ch, nullptr, nullptr_t, void>'
pfun<char const,&gl_fun_r_read_const_ch,nullptr> pf_read_const_ch;
                                                 ^
test_pmf.cpp:5:95: note: template is declared here
template <class T,T (*g)(),bool (*s)(T),class NP = std::nullptr_t,class dummy =
void>  struct pfun;"

If you uncomment the three commented lines and comment the three lines above it
( disregarding the space line ), the code compiles with no errors. Furthermore
if you compile in C++14 mode the code as shown there are no errors, and if you
compile with gcc-10.2 in C++14 or C++17 mode there are no errors. Using a 'char
const' rather than just a 'char' should not result in the code not working in
C++17.

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