[Bug c++/102721] New: out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[Bug c++/102721] New: out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

            Bug ID: 102721
           Summary: out-of-line member function definition fails to allow
                    lambda in unevaluated-context of  new feature in c++20
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nickhuang99 at hotmail dot com
  Target Milestone: ---

The following snippet code exposes that GCC parser doesn't comply with c++20
standard to allow lambda in unevaluated context. It seems this out-of-line
definition creates a new lambda type where lambda as operand of "decltype"
should be treated as in unevaluated context. So, it shouldn't create a new
lambda type in out-of-line definition.

Both clang and MSVC++ (https://www.godbolt.org/z/EMbb44fd8) parse this
correctly.
(with "-std=c++20")

template<class T>
struct A
{    
    void foo(const T){}
};

template<>
void A<decltype(+[]{})[3]>::foo(const decltype(+[]{})[3])
{}




<source>:9:6: error: ambiguating new declaration of 'void A<void (*
[3])()>::foo(void (* const*)())'
    9 | void A<decltype(+[]{})[3]>::foo(const decltype(+[]{})[3])
      |      ^~~~~~~~~~~~~~~~~~~~~
<source>:5:10: note: old declaration 'void A<T>::foo(T) [with T = void (*
[3])()]'
    5 |     void foo(const T){}
      |          ^~~

[Bug c++/102721] out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-10-12
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed,
Here is a better example of what is going wrong:
template<class T>
struct A
{    
    void foo(const T){}
};

using t = decltype(+[]{})[3];
using t1 = const decltype(+[]{})[3];

template<>
void A<t>::foo(t1)
{}

template<>
void A<decltype(+[]{})[4]>::foo(const decltype(+[]{})[4])
{}

The first template specialization is accepted but the second one is not.

[Bug c++/102721] out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> Confirmed,
> Here is a better example of what is going wrong:
> template<class T>
> struct A
> {    
>     void foo(const T){}
> };
> 
> using t = decltype(+[]{})[3];
> using t1 = const decltype(+[]{})[3];
> 
> template<>
> void A<t>::foo(t1)
> {}
> 
> template<>
> void A<decltype(+[]{})[4]>::foo(const decltype(+[]{})[4])
> {}
> 
> The first template specialization is accepted but the second one is not.
Note it is not array related either:
template<>
void A<decltype(+[]{})>::foo(const decltype(+[]{}))
{}

[Bug c++/102721] out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=92707

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Maybe related to PR 92707.

[Bug c++/102721] out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

--- Comment #4 from qingzhe huang <nickhuang99 at hotmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> Note it is not array related either:
> template<>
> void A<decltype(+[]{})>::foo(const decltype(+[]{}))
> {}

Yes, it is not array-related. But it is definitely member function issue with
out-of line definition. See this works fine:

template<class T>
void foo(const T){}

template<>
void foo<decltype(+[]{})>(const decltype(+[]{})); //declaration

template<>
void foo<decltype(+[]{})>(const decltype(+[]{}))
{}

[Bug c++/102721] out-of-line member function definition fails to allow lambda in unevaluated-context of new feature in c++20

[nickhuang99 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102721

--- Comment #5 from qingzhe huang <nickhuang99 at hotmail dot com> ---
(In reply to qingzhe huang from comment #4)
> (In reply to Andrew Pinski from comment #2)
> > Note it is not array related either:
> > template<>
> > void A<decltype(+[]{})>::foo(const decltype(+[]{}))
> > {}
> 
> Yes, it is not array-related. But it is definitely member function issue
> with out-of line definition. See this works fine:
> 
> template<class T>
> void foo(const T){}
> 
> template<>
> void foo<decltype(+[]{})>(const decltype(+[]{})); //declaration
> 
> template<>
> void foo<decltype(+[]{})>(const decltype(+[]{}))
> {}

Also interestingly, consider these following two cases:

1. template-class-member-function:
template<class T>
struct A{
void foo(const T){}
};

template<>
void A<decltype(+[]{})>::foo(const decltype(+[]{})){}

2. class-with-template-member-function:
struct A{
template<class T>
void foo(const T){}
};

template<>
void A::foo<decltype(+[]{})>(const decltype(+[]{})){}


Only case #1 has such issue.