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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: Unglaublich, dass das funktioniert:
Date: Sun, 31 Dec 2023 14:26:03 -0800
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Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> writes:

> Ben Bacarisse wrote:
>
>> Andrey Tarasevich <andreytarasevich@hotmail.com> writes:
>>
>>> On 12/22/23 2:12 AM, Ralf Goertz wrote:
>>>
>>>> Am Thu, 21 Dec 2023 18:13:39 -0800
>>>> schrieb Andrey Tarasevich <andreytarasevich@hotmail.com>:
>>>>
>>>>> On 12/20/23 11:34 PM, Ralf Goertz wrote:
>>>>>
>>>>>> On Wed, 20 Dec 2023 22:56:24 -0800
>>>>>> Andrey Tarasevich <andreytarasevich@hotmail.com> wrote:
>>>>>>     >>>>>> Meh... An old example of break-less switch-case branching
>>>>>>
>>>>>>>       #include <iostream>
>>>>>>>
>>>>>>>       int main(int argc, char **)
>>>>>>>       {
>>>>>>>         switch (argc)
>>>>>>>                if (0) case 1:  std::cout << 1 << std::endl;
>>>>>>>           else if (0) case 2:  std::cout << 2 << std::endl;
>>>>>>>           else if (0) case 3:  std::cout << 3 << std::endl;
>>>>>>>           else if (0) default:  std::cout << "whatever" << std::endl;
>>>>>>>       }
>>>>>>>
>>>>>>> As illustrated above, even if the body of switch is not enclosed
>>>>>>> into `{ ... }`, one can still stuff a lot into a single protracted
>>>>>>> statement, including multiple `case` labels.
>>>>>>
>>>>>> Interestingly enough, I get a warning with gcc:
>>>>>>
>>>>>> warning: statement will never be executed [-Wswitch-unreachable]
>>>>>>              if (0) case 1:  std::cout << 1 << std::endl;
>>>>>>
>>>>>> although of course I still see the output of ?1? when the program is
>>>>>> called without arguments.
>>>>>
>>>>> The compiler complains specifically about the `if (0)` part, not
>>>>> about the part that outputs 1.
>>>>
>>>> Hm, I obviously get that the `if (0)` condition can't ever be fulfilled,
>>>> but according to <https://en.cppreference.com/w/cpp/language/statements>
>>>> selection statements (1,2) the statement after the condition statement
>>>> is part of the if statement.  So part of the if statement may still be
>>>> executed.
>>>
>>> This has absolutely nothing to do with the fulfillability of if's
>>> condition.  You can replace `if (0)` with `if (1)` and you will still get
>>> the same warning.  The properties of that `if` are completely beside the
>>> point here.
>>
>> I don't find that to be the case.  gcc complains about this if (0) being
>> unreachable:
>>
>>    switch (argc)
>>        if (0) case 1: std::cout << 1 << "\n";
>>
>> but not this one:
>>
>>    switch (argc)
>>        if (1) case 1: std::cout << 1 << "\n";
>
> I think "if (1)" *is* equivalent to no-op here or, in AT's terms,
> cannot "potentially generate executable instructions".  I would say
> "there is no code to reach hence there is no switch-unreachable
> warning".

Some compilers complain.  Other don't.

I've seen the 'if(1)' version get a warning, and not get a
warning with a different compiler.

I've seen a 'for(;0;) case 1: ...' get a warning in some
circumstances, and not in others.

I tried some other variations, and got various results
under different compilers.

I'm reminded of Abraham Lincoln's review:  "For people who
like this sort of thing, this is the sort of thing that
those people like."