[Bug c++/94619] New: String literals as non-type template parameter fails to compile with partial specialization of calling function

[pacoarjonilla at yahoo dot es]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94619

            Bug ID: 94619
           Summary: String literals as non-type template parameter fails
                    to compile with partial specialization of calling
                    function
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pacoarjonilla at yahoo dot es
  Target Milestone: ---

I'm not sure if this is correct C++, but the compiler seems to be unable to 
deduce the template parameter of fu when it is a non-type kind.

If the code is not correct, this error blocks fu from being overloaded by more
specific templates :(

The diagnostic
"
mismatched types ‘A<length>’ and ‘A<...auto...>’
"
is suspicious of a bug in GCC

>>>>>>>>>>>>>>>>>>>>>>>>>>>


template <unsigned length> struct A {
    char str [length];

    constexpr A(char const (&s) [length]) {
        for (int i = 0; i < length; ++i)
            str[i] = s[i];
    }
};

template <A a> struct B {
    void bar() const {
        return a.str;
    }
};

//template <typename T> void fu (T    const& t) // Compiles successfully
  template <A        a> void fu (B<a> const& t) // Does not compile
//template <unsigned l> void fu (B<A<l>> const& t) // Does not compile either
{
      t.bar();
}

void test() {
    B<":/"> m;
    fu(m);
}


>>>>>>>>>>>>>>>>>>>>>>>>>>>

gcc10 --std=c++20 code.cc -c

meta.cc:17:37: error: class template argument deduction failed:
   17 |   template <A        a> void fu (B<a> const& t) // Does not compile
      |                                     ^
meta.cc:17:37: error: no matching function for call to ‘A(A<...auto...>)’
meta.cc:4:15: note: candidate: ‘template<unsigned int length> A(const char
(&)[length])-> A<length>’
    4 |     constexpr A(char const (&s) [length]) {
      |               ^
meta.cc:4:15: note:   template argument deduction/substitution failed:
meta.cc:17:37: note:   mismatched types ‘const char [length]’ and
‘A<...auto...>’
   17 |   template <A        a> void fu (B<a> const& t) // Does not compile
      |                                     ^
meta.cc:1:35: note: candidate: ‘template<unsigned int length> A(A<length>)->
A<length>’
    1 | template <unsigned length> struct A {
      |                                   ^
meta.cc:1:35: note:   template argument deduction/substitution failed:
meta.cc:17:37: note:   mismatched types ‘A<length>’ and ‘A<...auto...>’
   17 |   template <A        a> void fu (B<a> const& t) // Does not compile
      |                                     ^
meta.cc: In function ‘void fu(const int&)’:
meta.cc:20:9: error: request for member ‘bar’ in ‘t’, which is of non-class
type ‘const int’
   20 |       t.bar();
      |         ^~~
meta.cc: In function ‘void test()’:
meta.cc:25:9: error: no matching function for call to ‘fu(B<A<3>{":/"}>&)’
   25 |     fu(m);
      |         ^
meta.cc:17:30: note: candidate: ‘template<A<...auto...> a> void fu(const int&)’
   17 |   template <A        a> void fu (B<a> const& t) // Does not compile
      |                              ^~
meta.cc:17:30: note:   template argument deduction/substitution failed:
meta.cc:25:9: note:   couldn’t deduce template parameter ‘a’
   25 |     fu(m);
      |         ^