[Bug c++/52103] New: need a finer control over -Woverloaded-virtual

[mlg7 at yandex dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52103

             Bug #: 52103
           Summary: need a finer control over -Woverloaded-virtual
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: m...@yandex.ru


Let me first explain my use case.
I have a large existing non-gcc source which I am porting to a platform
supported by gcc. Due to the difference in dialects, I have to insert const
modifiers for the function parameters. But when I change the signature of a
base class by inserting const, it does not affect the derived classes --
instead, it just breaks inheritance. This is where -Woverloaded-virtual comes
to play.

Unfortunately, it generates too much noise, rendering the feature almost
unusable for this case. Indeed, the best place to hide an important message is
a hundred of useless messages of the same sort. (So this is a usability issue.)

Considering that -Woverloaded-virtual is rather doing what its name suggests, I
propose to implement two finer-tuned options.

-Woverloaded-modifiers
gives a warning when the two signatures differ only in modifiers like const.
(Consider also volatile and restrict.)
struct Base{ void f(Base& x){} };
struct Derived: public Base{ void f(const Base& x){} };

-Woverloaded-compatible
gives a warning when the two signatures differ in parameter classes, but one of
the classes is a descendant of the other.
struct First{ void g(Base&x){} };
struct Second{ void g(Derived&x){} };

I underline that -Woverloaded-modifiers and -Woverloaded-compatible should not
be limited to virtual functions: virtual could be implied if the function is
virtual in the base class. (You may disagree with me here, in this case I want
to hear your arguments.)

(I do not insist on namely these names, maybe you have some better
suggestion(s), but I insist on the functionality.)

As to -Woverloaded-virtual, I think, it should be left as it is.

Now some code example. Note that with -Woverloaded-modifiers
-Woverloaded-compatible there should be no warning for struct ThereIsNoProblem
because the argument types are unrelated.

$cat inher4.cpp 
#include <stdio.h>

struct Base{ void f(const Base& x){printf("Base.f()\n");} };
struct Derived: public Base{ void f(Base& x){printf("Derived.f()\n");} };

struct First{ virtual void g(Base&x){printf("First.g()\n");} };
struct Second:public First{ virtual void g(Derived&x){printf("Second.g()\n");}
};
struct ThereIsNoProblem:public First{ virtual void g(int
x){printf("ShouldBeValid.g()");} };

int main()
{
    Base x;
    Derived y;
    Base*py=&y;
    y.f(x);
    py->f(Base());
    Second z;
    First*pz=&z;
    //z.g(x); // error: no matching function for call to ‘Second::g(Base&)’
    z.g(y);
    pz->g(x);
    pz->g(y);
}
$g++ -Woverloaded-virtual inher4.cpp 
inher4.cpp:6:28: warning: ‘virtual void First::g(Base&)’ was hidden
inher4.cpp:7:42: warning:   by ‘virtual void Second::g(Derived&)’
inher4.cpp:6:28: warning: ‘virtual void First::g(Base&)’ was hidden
inher4.cpp:8:52: warning:   by ‘virtual void ThereIsNoProblem::g(int)’
$./a.out 
Derived.f()
Base.f()
Second.g()
First.g()
First.g()
$