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From: "psmith at gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109740] New: -Woverloaded-virtual is too aggressive
Date: Thu, 04 May 2023 16:40:36 +0000
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X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109740

            Bug ID: 109740
           Summary: -Woverloaded-virtual is too aggressive
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: psmith at gnu dot org
  Target Milestone: ---

I understand the impetus for the -Woverloaded-virtual warning but I think it
should be further constrained, or maybe broken into levels of severity that=
 can
be enabled.

The issue I'm running into is that a superclass has a virtual method signat=
ure
with some number of arguments, and the subclass has an overloaded method wi=
th a
different number of arguments.  For example:

  struct A { virtual void foo(int); };
  struct B : public A { void foo(); };

We do this kind of thing all the time because the subclass's version of this
method has more knowledge, or whatever, and doesn't need the extra argument=
s.

However, this fires the overloaded-virtual warning.  I don't think this
warrants a warning: if I call B.foo() then it will always do the right thin=
g.=20
If I call B.foo(10) it will give me an error.  There's no way that the comp=
iler
could ever call the wrong thing "behind my back" due to some sort of type
conversion that is not obvious from the code, because the number of argumen=
ts
are different: either it does exactly what I expect and there's no need for=
 a
warning, or I get an error and there's no need for a warning.

This subclassing capability is (IMO) useful and shouldn't be restricted, bu=
t I
would prefer to keep the warning for in other situations that could potenti=
ally
cause misbehavior.

Would it be possible to suppress this warning if the overload can't possibly
conflict due to number of arguments?

There is also the possibility that a subclass method has the same number of
arguments but they are of incompatible types which cannot be converted betw=
een.
 There is an argument to be made that this, also, shouldn't cause a warning=
.=20
I'm not sure how straightforward it is to determine "can't be converted"
although there are some obvious ones.=