Re: [PATCH] c++: Add new warning options for C++ language mismatches

[Jonathan Wakely <jwakely@redhat.com>]

On 19/05/21 12:53 -0400, Marek Polacek wrote:
>On Wed, May 19, 2021 at 05:39:24PM +0100, Jonathan Wakely via Gcc-patches wrote:
>> Jakub pointed out I'd forgotten the spaces before the opening parens
>> for function calls. The attached patch should fix all those, with no
>> other changes.
>>
>> Tested x86_64-linux. OK for trunk?
>
>Nice, this is cool.
>
>> --- a/gcc/c-family/c.opt
>> +++ b/gcc/c-family/c.opt
>> @@ -431,6 +431,22 @@ Wc++20-compat
>>  C++ ObjC++ Var(warn_cxx20_compat) Warning LangEnabledBy(C++ ObjC++,Wall)
>>  Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020.
>>
>> +Wc++11-extensions
>> +C++ ObjC++ Var(warn_cxx11_extensions) Warning LangEnabledBy(C++ ObjC++,Wall) Init(1)
>> +Warn about C++11 constructs in code compiled with an older standard.
>> +
>> +Wc++14-extensions
>> +C++ ObjC++ Var(warn_cxx14_extensions) Warning LangEnabledBy(C++ ObjC++,Wall) Init(1)
>> +Warn about C++14 constructs in code compiled with an older standard.
>> +
>> +Wc++17-extensions
>> +C++ ObjC++ Var(warn_cxx17_extensions) Warning LangEnabledBy(C++ ObjC++,Wall) Init(1)
>> +Warn about C++17 constructs in code compiled with an older standard.
>> +
>> +Wc++20-extensions
>> +C++ ObjC++ Var(warn_cxx20_extensions) Warning LangEnabledBy(C++ ObjC++,Wall) Init(1)
>> +Warn about C++20 constructs in code compiled with an older standard.
>> +
>
>So these are enabled by -Wall but also turned on by default?  Let's choose one
>and then drop either the Init(1) or the LangEnabledBy(C++ ObjC++,Wall) part?

Ah, good point. I mostly just cargo-cult what I see in that file (is
the format documented somewhere?)

I think to preserve the current behaviour (using these constructs in
an unsupported dialect warns by default) we want them to be Init(1)
but not in -Wall. Or we could change the behaviour, and include them
in -Wall and not Init(1), but then people who don't use -Wall (aka
idiots and beginners) would not get the warnings.

Any preference?