Re: GCC 9.2, partial specialization with non-type template parameters: bug or not?

[Bence Kodaj <bence.kodaj@gmail.com>]

Hi again,

After running my code through GCC 9.3 on godbolt, I can now answer my own
question :)  Turns out this was indeed a bug in GCC 9.2, and it's fixed in
GCC 9.3.

Best regards,
Bence Kodaj

On Sat, Apr 18, 2020 at 9:51 PM Bence Kodaj <bence.kodaj@gmail.com> wrote:

> Dear list members,
>
> I'd like to ask for your help with the following: in GCC 9.2., is partial
> template specialization expected to work for *all* allowed non-type
> template parameters of class type?
>
> For a concrete example, please see the code below: it prints "false,
> true", i.e., apparently, specializing T for X< Int > does not succeed,
> whereas it does for Y< int >. Since Int is now (in C++20) allowed to be
> used as the type of non-type template parameters, one would naively expect
> the specialization of T for X< Int > to succeed just like the one for Y<
> int > does. Is this belief justified, or am I missing something?
>
> Disclaimer: I do realize that C++20 support in GCC 9 is experimental - I'm
> just looking for clarity regarding this issue, i.e., does the C++20 (draft)
> standard require that the code below print "true, true"?
>
> Best regards,
> Bence Kodaj
> -------------
> - Compiler version (from g++ -v): gcc version 9.2.1 20191102 (Ubuntu
> 9.2.1-17ubuntu1~18.04.1)
> - Compilation command: g++ -std=c++2a main.cpp
>
> - Code:
> #include <iostream>
> #include <type_traits>
>
> struct Int {
>     constexpr Int(int v): value{ v } {}
>     const int value;
> };
>
> template< Int i >
> struct X {};
>
> template< int i >
> struct Y {};
>
> template< typename >
> struct T : std::false_type {};
>
> template< Int i >
> struct T< X< i > > : std::true_type {};
>
> template< int i >
> struct T< Y< i > > : std::true_type {};
>
> int main()
> {
>     std::cout << std::boolalpha << T< X< 123 > >::value << ", " << T< Y<
> 123 > >::value << "\n";
>     return 0;
> }
>