[Issue 12777] const/immutable member function violating its const-ness - confusing error message

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=12777

--- Comment #4 from Ivan Kazmenko <gassa at mail.ru> ---
(In reply to bearophile_hugs from comment #3)
> What error message do you suggest? Perhaps something like this?
> 
> test.d(3): Error: const methods (like 'S.fun') cannot modify instance
> members (like 'S.v'), only constructors can initialize them
> test.d(4): Error: immutable methods (like 'S.gun') cannot modify instance
> members (like 'S.v'), only constructors can initialize them

The part about constructors is formally false: not only constructors can
initialize a non-const instance member.

Here's a C++ variant and the respective errors for inspiration:

-----
struct S {
    int v;
    void fun () const {v++;}
};
class C {
    int v;
    void fun () const {v++;}
};
int main (void) {}
-----

GCC - g++ errors:

-----
const_member.cpp: In member function 'void S::fun() const':
const_member.cpp:3:25: error: increment of member 'S::v' in read-only object
     void fun () const {v++;}                         ^
const_member.cpp: In member function 'void C::fun() const':
const_member.cpp:7:25: error: increment of member 'C::v' in read-only object
     void fun () const {v++;}
                         ^
-----

CLang - clang++ errors:

-----
const_member.cpp:3:25: error: read-only variable is not assignable
    void fun () const {v++;}
                       ~^
const_member.cpp:7:25: error: read-only variable is not assignable
    void fun () const {v++;}
                       ~^
-----

Visual Studio 12 - cl.exe errors:

-----
const_member.cpp(3) : error C3490: 'v' cannot be modified because it is being
accessed through a const object
const_member.cpp(7) : error C3490: 'v' cannot be modified because it is being
accessed through a const object
-----

I'd say Visual Studio's message is the most readable.

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