[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dcb314 at hotmail dot com

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 94768 has been marked as a duplicate of this bug. ***

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[kdevel at vogtner dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

Stefan <kdevel at vogtner dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kdevel at vogtner dot de

--- Comment #8 from Stefan <kdevel at vogtner dot de> ---
(In reply to Jonathan Wakely from comment #4)
> There are cases which are either not diagnosable or cannot be
> written, such as this example from Doug Gregor:
> 
> in generic code, there might not be a way to create a value with the
> appropriate type. For example:
> 
>        template<typename T>
>        T maybe_call(std::function<T(void)> f) {
>                if (f)
>                        return f();
>                else
>                        abort_program();
> 
>                // Cannot write a return here, because we have no way to
> create a value of type 'T'
>        }

One can of course write a return there:

   template<typename T>
   T maybe_call(std::function<T(void)> f)
   {
      if (f)
         return f();
      else
         abort_program();

      // Cannot write a return here, because we have no way to create a value
of type 'T'
      return f();
   }

which refactors nicely into

   template<typename T>
   T maybe_call(std::function<T(void)> f)
   {
      if (! f)
         abort_program();
      return f();
   }

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |contact@thunderperfectwitch
                   |                            |craft.org

--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 109364 has been marked as a duplicate of this bug. ***

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[david at rothlis dot net]

------- Comment #6 from david at rothlis dot net  2010-04-30 14:01 -------
OK, thanks for the explanation!


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[redi at gcc dot gnu dot org]

------- Comment #5 from redi at gcc dot gnu dot org  2010-04-30 11:26 -------
The point being that it's undefined behaviour to /return/ from such a function,
not to /write/ such a function.

If the programmer guarantees the "missing return" will never happen then
there's no error.

I was wrong to say it's OK as long as the caller doesn't use the return value -
but it is OK as long as control never reaches the "missing return"


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[redi at gcc dot gnu dot org]

------- Comment #4 from redi at gcc dot gnu dot org  2010-04-30 11:22 -------
(In reply to comment #3)
> > a function with a missing return is valid
> 
> I just can't reconcile that with the following line from the C++ standard: "It
> is now invalid to return (explicitly or implicitly) from a function which is
> declared to return a value without actually returning a value."  Any insights?

Apologies, you're right, in C++ it's not valid and is always undefined
behaviour.

But there have been long discussions on the standard committee reflector about
why it cannot be a "diagnostic required" error, rather than undefined
behaviour.  There are cases which are either not diagnosable or cannot be
written, such as this example from Doug Gregor:

in generic code, there might not be a way to create a value with the
appropriate type. For example:

       template<typename T>
       T maybe_call(std::function<T(void)> f) {
               if (f)
                       return f();
               else
                       abort_program();

               // Cannot write a return here, because we have no way to create
a value of type 'T'
       }


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[david at rothlis dot net]

------- Comment #3 from david at rothlis dot net  2010-04-30 06:05 -------
> a function with a missing return is valid

I just can't reconcile that with the following line from the C++ standard: "It
is now invalid to return (explicitly or implicitly) from a function which is
declared to return a value without actually returning a value."  Any insights?

Thanks for the "-Werror=return-type" tip.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[redi at gcc dot gnu dot org]

------- Comment #2 from redi at gcc dot gnu dot org  2010-04-29 20:36 -------
that should be -Werror=return-type 

but spelling aside, this is not a bug now

the compiler MUST NOT reject it by default, since a function with a missing
return is valid as long as the caller does not use the (missing) return value. 
making this an unconditional error would be non-conforming

if you want that behaviour, use -Werror=return-type


-- 

redi at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943

[Bug c++/43943] "warning: no return statement in function returning non-void" should be an error

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2010-04-29 20:31 -------
-Werror=Wreturn-type will turn just that warning into an error message in
recent GCC's.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43943