[Bug c++/88355] New: [c++20] Placeholder non-type template argument type deduction fails with custom types

[emmanuel.le-trong@cnrs-orleans.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88355

            Bug ID: 88355
           Summary: [c++20] Placeholder non-type template argument type
                    deduction fails with custom types
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: emmanuel.le-trong@cnrs-orleans.fr
  Target Milestone: ---

Deducing the type of a non-type template parameter containing a placeholder
fails with custom types. 

$ g++-9 -v                                                                      
Using built-in specs.
COLLECT_GCC=g++-9
COLLECT_LTO_WRAPPER=/home/manu/system/opt/gcc-9/libexec/gcc/x86_64-pc-linux-gnu/9.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-9/configure --prefix=/home/manu/system/opt/gcc-9
--program-suffix=-9 : (reconfigured) ../gcc-9/configure
--prefix=/home/manu/system/opt/gcc-9 --program-suffix=-9
Thread model: posix
gcc version 9.0.0 20181127 (experimental) (GCC)

$ cat bug_2.cpp
    #include <type_traits> // for std::is_same_v

    struct T {};

    template <auto value>
    struct U {};

    template <auto value> 
    void f (U <value>) {}

    int main ()
    {
        constexpr T t;
        f    (U<1>{});  // OK
        f<t> (U<t>{});  // OK
        // This works: 
        auto x = t;
        static_assert (std::is_same_v <decltype (x), T>); // OK
        // This should work as well, as per [temp.arg.nontype], see below
        f    (U<t>{});  // Error
    }

$ g++-9 -std=c++2a -Wall -Wextra    bug_2.cpp   -o bug_2
bug_2.cpp: In function ‘int main()’:
bug_2.cpp:20:21: error: no matching function for call to ‘f(U<T()>)’
   20 |         f    (U<t>{});  // Error
      |                     ^
bug_2.cpp:9:10: note: candidate: ‘template<auto value> void f(U<value>)’
    9 |     void f (U <value>) {}
      |          ^
bug_2.cpp:9:10: note:   template argument deduction/substitution failed:
bug_2.cpp:20:21: note:   mismatched types ‘T’ and ‘const T’
   20 |         f    (U<t>{});  // Error
      |                     ^
make: *** [<builtin>: bug_2] Error 1


The standardese in the latest draft is:

---8<---

12.3.2 Template non-type arguments [temp.arg.nontype]
If the type T of a template-parameter (12.1) contains a placeholder type
(9.1.7.5) or a placeholder for a deduced class type (9.1.7.6), the type of the
parameter is the type deduced for the variable x in the invented declaration

    T x = template-argument ;

--->8---