[Bug c++/114410] New: compiler issue in reference_binding when using rvalue reference to member of const object

[Bug c++/114410] New: compiler issue in reference_binding when using rvalue reference to member of const object

[cvoica at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

            Bug ID: 114410
           Summary: compiler issue in reference_binding when using rvalue
                    reference to member of const object
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cvoica at gmail dot com
  Target Milestone: ---

The issue "internal compiler error: in reference_binding, at cp/call.cc:2020"
is visible when compiling this program

#include<iostream>
#include<string>

template<typename T>
class Holder{
    typedef T value_type;
    value_type m_v;
public:
    Holder(const value_type& v):m_v(v){}
    operator const value_type() const{return m_v;}
    operator value_type(){return m_v;}

//    operator const value_type&() const{return m_v;}
//    operator value_type&(){return m_v;}
};

struct A {
    Holder<std::string> m_i;
    A(const std::string& i):m_i(i){}
};


void bad_const_access(const A* a){
    std::cout << (std::string&)a->m_i << std::endl; //crash in gcc-13 but OK in
gcc-12
}
void correct_const_access(const A* a){
    std::cout << (const std::string&)a->m_i << std::endl;
}
int main(){
    const A a("test");
    bad_const_access(&a);
    correct_const_access(&a);
    return 0;
}

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|https://godbolt.org/#z:OYLg |
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--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1DIApACYAQuYukl9ZATwDKjdAGFUtAK4sGe1wAyeAyYAHI%2BAEaYxCAAzACspAAOqAqETgwe3r56KWmOAkEh4SxRMQm2mPYFDEIETMQEWT5%2BXJXVGXUNBEVhkdFxiQr1jc05bcPdvSVlgwCUtqhexMjsHOaxwcjeWCaxbk7DxJise9gmGgCCG1s7mHtuR8HAZxfXVwSYLEkGnw8EAE8koxWJgANQAFVeV22TAUCjBAAlPFhiCYAOxWK5gnFgwHArBUSFggBuYi8mAA%2Bvj7rEsZdcaTyVSaWCWJSSXt6UkvBFaHhkCA3ozkbRURA0AxhkzvCygfczAA2UlzEDskkQElzDFWdEAEWFuNQwOITCIxDBkulZNl1PlEDmloEwx1xwIywYbI5XIxBuxRpNZpIMopduBDtdmHdxE96p9%2BrebwA9EnGcbokGLVaCCG5cDzIqHU6pQRI9HY966b7k6mAxnzbmwwrC9rMW6PV7OVWE1dq1Xe1cjl4HGCGTrDTjRaiHsN0CAQE8jGcvXgff6cZcJc6c7P54uXkqwXhVey8BBj%2BOe9cE/33neSag8OgwREmOhKdnKUxkKt4VuS6OABUYJMK29KMruKBLDmDwPGCECQfuBZzEwAC0ZynmCsH7GCkGuLQXJgimyCmgoCBHp6wA/qhXCxC%2BXg5gA8gA0hRYJUcgNFmImV4Pk%2BTrEMcDgftuX4/pgf7ZkBIFgROuEEHOUEMVh%2Bxwf%2B0qIQQxDPMhaEYZSeAqW4cF4Qw6AEbe1ZXMEOYsEwwQRpiclSQyTAQOYZifC6ZhmNqt6Mq%2B76ft%2Bv4KO5SqgWuDK4mggmYMJwXiX%2BBaRf5uLtjGYIaFFVmXBwCy0Jw8S8H4HBaKQqCcMZljWLhSwrOCGw8KQBCaPlCwANYgJIkgAHQ9QNg1DfonCSCVbUVZwvAKCAGgtW1CxwLASBoN8dDROQlCrUk60xMAtFtDQtCfMQM0QBEE0RMEDQApd13EACjERNo8WtdwvCrWwgiMQwtC3WVvBYBEXjAG4Yi0DN72kFgdlGOIAPQ3gQmOCSEkTZgqjxQxazlTZVQTfyESmg9HhYBNWl4CwC18AYwAKAAangmAAO6McCpXNfwggiGI7BSDIgiKCo6gI7obQGEYKDWNY%2Bh4BEM2QAsxo1JDqGznseqmDVlhmBoYKoQA6uD%2BsGxjWlMCbBKGI4yC8KgqOCU%2BaOK%2B0r0ZC4ZljK0pCBMEfSlAMbR5OkAhe7kqQhww0z9DEExVG7AhdKMngtHodgJ7UIw9H7MyB7YWdhxMWfRwHscLAo9WrBIBVFeNCOVRwYKqAAHIqqGKpI7E/mCtG9Vw8G4IQwZNXMvBvVocydSAPm9T5c/zwviSFRwY2kKV5UN9Ns3zQDi0wIgIDQTyBCbRAmD4OaT56FzwiiOI/M30LagTWLpDM6aSScDwNccMVa8TQ3RiDFj5glQESFubcO5d2QD3WIfd4IeDWvQC0I8x4LSnrEOBmDsE4NwSNFedcN5TVsNvce7V8FmEIXbYhZDJ6kAdmkZwkggA%3D

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
dup.

*** This bug has been marked as a duplicate of bug 113141 ***

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[cvoica at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

--- Comment #3 from Cristian VOICA <cvoica at gmail dot com> ---
be aware that is initial issue mentions "If the type of r is changed to const
int&, the program correctly compiles with GCC"
I have this in my code as well but this is not always what programmer wants as
it creates a reference to a temporary object and extend the lifetime with const
not aan lvalue const reference.
If possible, a warning would be nice here

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[cvoica at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

--- Comment #4 from Cristian VOICA <cvoica at gmail dot com> ---
I'm also trying to understand another thing. 
Perhaps you can point me to where to read more about it as I was not expecting
casting to a non-const reference to work for a member in a const object.

Is there a warning for (std::string&)a->m_i on "const A* a" ?

Thx!

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
No, because a (T) style cast can do almost anything. In this case it's
equivalent to

const_cast<std::string&>(static_cast<const std::string&>(a->m_i))

which is valid code.

-Wold-style-cast will warn about using any form of (T) cast, which can
encourage you to be more careful and precise about what you're trying to do.

If you had written it like this:

std::cout << static_cast<std::string&>(a->m_i) << std::endl;

Then it wouldn't even compile.

tl;dr don't use C-style casts if you want the compiler to tell you when you're
doing something wrong.

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[cvoica at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

--- Comment #6 from Cristian VOICA <cvoica at gmail dot com> ---
makes sense, thx!

[Bug c++/114410] compiler issue in reference_binding when using rvalue reference to member of const object

[cvoica at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114410

--- Comment #7 from Cristian VOICA <cvoica at gmail dot com> ---
I've found a flag that sounds closer to my needs: -Wcast-qual
As the issue seems to me to come from the need to stay compatible with C I
think this flag does a pretty decent job to hint at the problem.

<source>: In function 'void bad_const_access(const A*)':
<source>:24:18: warning: cast from type 'const
Holder<std::__cxx11::basic_string<char> >*' to type 'std::string*' {aka
'std::__cxx11::basic_string<char>*'} casts away qualifiers [-Wcast-qual]
   24 |     std::cout << (std::string&)a->m_i << std::endl; //crash in gcc-13
but OK in gcc-12
      |                  ^~~~~~~~~~~~~~~~~~~~
<source>:24:35: internal compiler error: in reference_binding, at
cp/call.cc:2020
   24 |     std::cout << (std::string&)a->m_i << std::endl; //crash in gcc-13
but OK in gcc-12
      |                                   ^~~