[Bug c++/97740] New: Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[Bug c++/97740] New: Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[markus.boeck02 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

            Bug ID: 97740
           Summary: Weird error message about accessing a private member
                    of my own class inside of std::string_view inside of
                    constexpr
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus.boeck02 at gmail dot com
  Target Milestone: ---

Created attachment 49513
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49513&action=edit
Reproduction of the bug. Simply compile with -std=c++17

I want to apologize for the code you are about to see already. I am one of
those people who likes to constexpr and templates at times. In one of my recent
code that does some constexpr string parsing and validation I encountered a
very very weird error message:

/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/string_view: In
instantiation of
'cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)> [with auto:7 = {const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&}]':
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/type_traits:2506:26:  
required by substitution of 'template<class _Fn, class ... _Args> static
std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)),
std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn =
cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>; _Args = {const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&}]'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/type_traits:2517:55:  
required from 'struct std::__result_of_impl<false, false,
cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&>'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/type_traits:138:12:  
recursively required by substitution of 'template<class _Result, class _Ret>
struct std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename
_CTp::type> > [with _Result =
std::__invoke_result<cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&>; _Ret = void]'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/type_traits:138:12:  
required from 'struct
std::__and_<std::__is_invocable_impl<std::__invoke_result<cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&>, void, true, void>,
std::__call_is_nothrow<std::__invoke_result<cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&>,
cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&> >'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/type_traits:2979:12:  
required from 'struct
std::is_nothrow_invocable<cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::pair<cld::detail::CommandLine::Pointer<(& detail::INCLUDESbind0)>,
std::in_place_type_t<std::basic_string_view<char, std::char_traits<char> > >
>&>'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/tuple:1715:37:   required
from 'constexpr const bool std::__unpack_std_tuple<template<class _Fn, class
... _ArgTypes> struct std::is_nothrow_invocable,
cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>, const
std::tuple<std::pair<cld::detail::CommandLine::Pointer<(&
detail::INCLUDESbind0)>, std::in_place_type_t<std::basic_string_view<char,
std::char_traits<char> > > > >&>'
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/tuple:1730:14:   required
from 'constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn =
cld::detail::CommandLine::parseOptions<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>, {detail::INCLUDESbind0,
std::in_place_type<std::basic_string_view<char, std::char_traits<char> >
>}>::<lambda(auto:7&& ...)>; _Tuple = const
std::tuple<std::pair<cld::detail::CommandLine::Pointer<(&
detail::INCLUDESbind0)>, std::in_place_type_t<std::basic_string_view<char,
std::char_traits<char> > > > >&]'
<source>:456:36:   required from 'constexpr auto
cld::detail::CommandLine::parseOptions(std::string_view, cld::CLIMultiArg)
[with T = cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>; auto& ...args =
{detail::INCLUDESbind0, std::in_place_type<std::basic_string_view<char,
std::char_traits<char> > >}; std::string_view = std::basic_string_view<char>]'
<source>:502:62:   required from here
/opt/compiler-explorer/gcc-10.2.0/include/c++/10.2.0/string_view:287:34: error:
'char cld::Constexpr::basic_fixed_string<char, 7>::m_data [7]' is private
within this context
  287 |  return basic_string_view{_M_str + __pos, __rlen};
      |                           ~~~~~~~^~~~~~~
<source>:18:5: note: declared private here
   18 |   T m_data[N] = {};
      | 

I tried to create a minimum reproducible from scratch but failed to so. Instead
I now used my code and tried to minimize that as much as possible which sadly
still amounts to 500 lines of code but does reproduce the issue. I have
attached the file.

To my knowledge the code is correct and clang and MSVC compile it without
issues. Changing m_data in basic_fixed_string from private to public makes the
build succeed. Every version of GCC 8 to trunk fail with the same error. GCC 7
instead has an internal compiler error.

Alternatively see this compiler explorer link: https://godbolt.org/z/snnfo7

[Bug c++/97740] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|diagnostic                  |rejects-valid
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-11-09

[Bug c++/97740] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. the "diagnostic" keyword is for poor quality diagnostics, not ones that
shouldn't happen at all. Code that shouldn't give an error at all should use
"rejects-valid" instead.

[Bug c++/97740] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The trunk produces:
Output of x86-64 gcc (trunk) (Compiler #2)
<source>: In instantiation of
'cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)> [with auto:6
= {const cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&,
const cld::detail::CommandLine::Arg&}]':
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/type_traits:2555:26:
  required by substitution of 'template<class _Fn, class ... _Args> static
std::__result_of_success<decltype (declval<_Fn>()((declval<_Args>)()...)),
std::__invoke_other> std::__result_of_other_impl::_S_test(int) [with _Fn =
cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>; _Args =
{const cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&,
const cld::detail::CommandLine::Arg&}]'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/type_traits:2566:55:
  required from 'struct std::__result_of_impl<false, false,
cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&>'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/type_traits:151:12:
  recursively required by substitution of 'template<class _Result, class _Ret>
struct std::__is_invocable_impl<_Result, _Ret, true, std::__void_t<typename
_CTp::type> > [with _Result =
std::__invoke_result<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&>; _Ret = void]'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/type_traits:151:12:
  required from 'struct
std::__and_<std::__is_invocable_impl<std::__invoke_result<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&>, void, true, void>,
std::__call_is_nothrow<std::__invoke_result<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&>,
cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&> >'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/type_traits:3054:12:
  required from 'struct
std::is_nothrow_invocable<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
cld::detail::CommandLine::Arg&, const cld::detail::CommandLine::Arg&, const
cld::detail::CommandLine::Arg&>'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/tuple:1872:37:  
required from 'constexpr const bool std::__unpack_std_tuple<template<class _Fn,
class ... _ArgTypes> struct std::is_nothrow_invocable,
cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>, const
std::tuple<cld::detail::CommandLine::Arg, cld::detail::CommandLine::Arg,
cld::detail::CommandLine::Arg>&>'
/opt/compiler-explorer/gcc-trunk-20210828/include/c++/12.0.0/tuple:1887:14:  
required from 'constexpr decltype(auto) std::apply(_Fn&&, _Tuple&&) [with _Fn =
cld::detail::CommandLine::Pack<detail::INCLUDESarg0, detail::INCLUDESarg1,
detail::INCLUDESarg2>::parseOptionsImpl()::<lambda(auto:6&& ...)>; _Tuple =
const std::tuple<cld::detail::CommandLine::Arg, cld::detail::CommandLine::Arg,
cld::detail::CommandLine::Arg>&]'
<source>:408:49:   required from 'static constexpr auto
cld::detail::CommandLine::Pack<args>::parseOptionsImpl() [with const auto&
...args = {detail::INCLUDESarg0, detail::INCLUDESarg1, detail::INCLUDESarg2}]'
<source>:444:49:   required from 'constexpr const auto
cld::detail::CommandLine::parsedTuple<cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2> >'
<source>:458:48:   required from 'constexpr auto
cld::detail::CommandLine::parseOptions(std::string_view, cld::CLIMultiArg)
[with T = cld::detail::CommandLine::Pack<detail::INCLUDESarg0,
detail::INCLUDESarg1, detail::INCLUDESarg2>; auto& ...args =
{detail::INCLUDESbind0, std::in_place_type<std::basic_string_view<char,
std::char_traits<char> > >}; std::string_view = std::basic_string_view<char>]'
<source>:502:5:   required from here
<source>:411:31: error: too many initializers for
'std::array<std::basic_string_view<char>, 3>'
  411 |                 input.value...};
      |                               ^


clang accepts the code using GCC's libstdc++ while rejects it using LLVM's
libc++:
<source>:410:20: error: implicit instantiation of undefined template
'std::array<std::string_view, 3>'
            return std::array<std::string_view, sizeof...(input)>{
                   ^

[Bug c++/97740] [10/11/12/13 Regression] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.5
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=96716
            Summary|Weird error message about   |[10/11/12/13 Regression]
                   |accessing a private member  |Weird error message about
                   |of my own class inside of   |accessing a private member
                   |std::string_view inside of  |of my own class inside of
                   |constexpr                   |std::string_view inside of
                   |                            |constexpr
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Here's a reduced C++11 testcase that became rejects-valid after
r10-6416-g8fda2c274ac66d:

struct A {
  constexpr const int* get() const { return &m; }
private:
  int m = 42;
} a;

struct B { const int* p; };

template<class>
void f() {
  constexpr B b = {a.get()};
}

template void f<int>();

<stdin>: In function ‘void f()’:
<stdin>:11:25: error: ‘int A::m’ is private within this context
<stdin>:4:7: note: declared private here
<stdin>: In instantiation of ‘void f() [with <template-parameter-1-1> = int]’:
<stdin>:14:22:   required from here
<stdin>:11:25: error: ‘int A::m’ is private within this context
<stdin>:4:7: note: declared private here

[Bug c++/97740] [10/11/12/13 Regression] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
A C++14 version that's more similar to the original testcase using a generic
lambda and dependent initializer:

struct A {
  constexpr const int* get() const { return &m; }
private:
  int m = 42;
} a;

struct B { const int* p; };

template<A* arg>
void f() {
  [] (auto) {
    constexpr B b = {arg->get()};
  }(0);
}

template void f<&a>();

[Bug c++/97740] [11/12/13/14 Regression] Weird error message about accessing a private member of my own class inside of std::string_view inside of constexpr

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97740

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.