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Explicit instantiation of a class with concept'ed methods

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  Explicit instantiation of a class with concept'ed methods Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-19 13:33 +0200
    Re: Explicit instantiation of a class with concept'ed methods Bonita Montero <Bonita.Montero@gmail.com> - 2021-09-19 18:03 +0200

#81326 — Explicit instantiation of a class with concept'ed methods

Look at this code:

#include <concepts>

using namespace std;

struct A
{
     void op();
};

struct B
{
     void op( bool );
};

template<typename T>
concept concept_C = is_same<T, A>::value || is_same<T, B>::value;

template<typename T>
     requires concept_C<T>
struct C
{
     void opA()
         requires is_same<T, A>::value;
     void opB( bool f )
         requires is_same<T, B>::value;
     T *pt;
};

template<typename T>
     requires concept_C<T>
void C<T>::opA()
     requires is_same<T, A>::value
{
     pt->op();
}

template<typename T>
     requires concept_C<T>
void C<T>::opB( bool f )
     requires is_same<T, B>::value
{
     pt->op( f );
}

template
struct C<A>;

template
struct C<B>;

MSVC will instantiate opA for A as well as B and opB
also for A as well as B and gives the following errors:

(42,10): error : too many arguments to function call, expected 0, have 1
(46,8): message : in instantiation of member function 'C<A>::opB' 
requested here
(7,7): message : 'op' declared here
(34,9): error : too few arguments to function call, expected 1, have 0
(49,8): message : in instantiation of member function 'C<B>::opA' 
requested here
(12,7): message : 'op' declared here

clang gives similar errors, but g++12 is smarter and skips inappropriate
compilation, i.e. filters the functions through their concepts, i.e. it
compiles opA only for A and opB only for B - so which compiler is right
here ? And is there a workaround ?

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#81330

I wrote the same thing on Stack Overflow. C<A>::opB( bool )
and C<B>::opA() shouldn't be instantiated according to the
standard. So this is a bug of clang. MSVC has fixed the bug
in the latest release.

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