Cant seen to find the change in gcc13.2 release notes or error: invalid redeclaration of template<typename> with different constraints

Question

I had the following issue a couple of days ago when using arm-none-eabi-gcc toolchain on arch linux, which uses gcc13.2 as the gcc version, unlike the toolchain I was using in Ubuntu, I assumed it would be some version stuff so I went to the gcc release page to read the notes: https://gcc.gnu.org/gcc-13/changes.html, but as you will see nothing about redeclaration of templates is mentioned

upon some research I came to this reproducible example:

// compiles fine under gcc 12.3

#include <concepts>
#include <type_traits>

 template<typename T>
 class Sensor{
   public:
    int read();
  };

template<typename T>
  requires std::is_integral_v<T>
int Sensor<T>::read(){
 return 1;
}

int main(){
 Sensor<int>  s;
 s.read();
}


//does not compile under gcc 13.2
#include <concepts>
#include <type_traits>

template<typename T>

class Sensor{
 public:
    int read();
};

template<typename T>
 requires std::is_integral_v<T>
int Sensor<T>::read(){
 return 1;
}

int main(){
 Sensor<int>  s;
 s.read();
}

Could someone explain why in version 12.3 it is allowed but not in 13.2? Also I'd assume the constrainst enforced in 13.2 should have been a standard de facto way ago

Answer 1

The relevant note is in gcc/cp/ChangeLog:

2023-03-14  Patrick Palka  <...>

        PR c++/96830
        * pt.cc (push_inline_template_parms_recursive): Set
        TEMPLATE_PARMS_CONSTRAINTS.
        (push_template_decl): For an out-of-line declaration, verify
        constraints for each enclosing template scope match those of the
        original template declaratation.


    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-class5.C: New test.
            * g++.dg/cpp2a/concepts-class5a.C: New test.

Could someone explain why in version 12.3 it is allowed but not in 13.2?

12.3 was released before this bugfix was submitted.

---

The full commit message is:

commit cd5baeb4489b6a953abbc7f02fea457fd9ed2f83
Author: Patrick Palka <...>
Date:   Tue Mar 14 19:14:29 2023 -0400

    c++: redeclaring member of constrained class template [PR96830]

    An out-of-line definition of a member of a constrained class template
    needs to repeat the template's constraints, but it turns out we don't
    verify anywhere that the two sets of constraints match.  This patch
    adds such a check to push_template_decl, nearby a similar consistency
    check for the template parameter list lengths.

            PR c++/96830

    gcc/cp/ChangeLog:

            * pt.cc (push_inline_template_parms_recursive): Set
            TEMPLATE_PARMS_CONSTRAINTS.
            (push_template_decl): For an out-of-line declaration, verify
            constraints for each enclosing template scope match those of the
            original template declaratation.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-class5.C: New test.
            * g++.dg/cpp2a/concepts-class5a.C: New test.