[Bug c++/53281] New: poor error message for calling a non-const method from a const object

[Bug c++/53281] New: poor error message for calling a non-const method from a const object

[rui.maciel at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

             Bug #: 53281
           Summary: poor error message for calling a non-const method from
                    a const object
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rui.maciel@gmail.com


Consider the following code:

<code>
class Foo {
        void bar1() {}
        void bar2(Foo const &foo) {
                foo.bar1();
        }   
};

int main()
{
        return 0;
}
</code>

When this code is compiled with g++, the following error message is shown:

<message>
main.c++: In member function ‘const void Foo::bar2(const Foo&)’:
main.c++:4:26: error: passing ‘const Foo’ as ‘this’ argument of ‘const void
Foo::bar1()’ discards qualifiers [-fpermissive]
</message>

The error message is technically correct.  Yet, it is too cryptic and a bit
unhelpful, to the point it may be considered that the point of this error
message is entirely missed.  

As an alternative, it would be nice if g++ displayed an error message which
would actually be straight-forward and provided a clear description of the
problem at hand, such as "trying to call a non-const method from a const
object".

[Bug c++/53281] poor error message for calling a non-const method from a const object

[rui.maciel at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

--- Comment #1 from Rui Maciel <rui.maciel at gmail dot com> 2012-05-08 13:27:24 UTC ---
The same suggestion applies to the cases where a non-const method is called
from a const method, such as in the example below:

<code>
class Foo {
        void bar1() {}
        void bar2() const {
                bar1();
        }
};

int main()
{
        return 0;
}
</code>

The same error message is returned:
<message>
main.c++: In member function ‘void Foo::bar2() const’:
main.c++:4:22: error: passing ‘const Foo’ as ‘this’ argument of ‘void
Foo::bar1()’ discards qualifiers [-fpermissive]
</message>

[Bug c++/53281] poor error message for calling a non-const method from a const object

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-05-09
     Ever Confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-05-09 15:59:09 UTC ---
confirmed (but any change to the diagnostic should not mention the word 
"method" because C++ has member functions not methods)

[Bug c++/53281] poor error message for calling a non-const method from a const object

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.3                         |9.4

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 9.3.0 has been released, adjusting target milestone.

[Bug c++/53281] poor error message for calling a non-const method from a const object

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |11.0

[Bug c++/53281] poor error message for calling a non-const method from a const object

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
   Target Milestone|11.0                        |---
           Assignee|redi at gcc dot gnu.org            |unassigned at gcc dot gnu.org

[Bug c++/53281] poor error message for calling a non-const method from a const object

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

Barry Revzin <barry.revzin at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |barry.revzin at gmail dot com

--- Comment #14 from Barry Revzin <barry.revzin at gmail dot com> ---
Following up on this (and a lot of the ideas presented here are I think better
than "passing [...] discards qualifiers")...

This:

struct B {
    void f();
}; 

struct D : B {
    void const_f() const {
        f();
    }
};

gives me that same error:

<source>:8:10: error: passing 'const D' as 'this' argument discards qualifiers
[-fpermissive]
    8 |         f();
      |         ~^~
<source>:2:10: note:   in call to 'void B::f()'
    2 |     void f();
      |          ^

But this slight difference (D is now a template, though B is a non-dependent
base):

struct B {
    void f();
}; 

template <class T>
struct D : B {
    void const_f() const {
        f();
    }
};

instead emits: 

<source>:8:10: error: cannot convert 'const D<T>*' to 'B*'
    8 |         f();
      |         ~^~
<source>:2:10: note:   initializing argument 'this' of 'void B::f()'
    2 |     void f();
      |          ^

Which is... technically correct, that is the problem, but now that I'm used to
seeing the "discards qualifiers" error, this one really threw me for a loop. 

It'd be nice at the very least if these two examples gave the same error

[Bug c++/53281] poor error message for calling a non-const method from a const object

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53281

--- Comment #15 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 53035
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53035&action=edit
Incomplete patch