Re: [PATCH] gdbsupport: add support for references to checked_static_cast

[Simon Marchi <simon.marchi@efficios.com>]

On 5/24/23 09:22, Lancelot SIX wrote:
> Hi Simon,
> 
>> diff --git a/gdbsupport/gdb-checked-static-cast.h b/gdbsupport/gdb-checked-static-cast.h
>> index bc75244bddd0..7e5a69a6474d 100644
>> --- a/gdbsupport/gdb-checked-static-cast.h
>> +++ b/gdbsupport/gdb-checked-static-cast.h
>> @@ -66,6 +66,22 @@ checked_static_cast (V *v)
>>    return result;
>>  }
>>  
>> +/* Same as the above, but to cast from a reference type to another.  */
>> +
>> +template<typename T, typename V>
>> +T
>> +checked_static_cast (V &v)
>> +{
>> +  static_assert (std::is_reference<T>::value, "target must be a reference type");
> 
> Have you considered incorporating this constraint inside
> checked_static_cast's signature?  It could look like something like
> this:
> 
>     template<typename T, typename V>
>     typename std::enable_if<std::is_reference<T>::value, T>::type
>     checked_static_cast (V &v)
>     {
>       ...
>     }

I like static_assert because it's easier to write and read, and I find
that the error messages it produces are easier to understand.  They
point you directly to the static_assert line that says "target must be a
reference", so you know instantly what is wrong.  With template
parameters, you have to decrypt the template substitution message, it's
really not obvious.  However...

> If T is not a reference, then this template won't be instantiated at all
> instead of instantiating it and having an error.  This could allow other
> templates / functions to be considered if they matched instead of just
> producing an error.

... it seems like there are some pragmatic advantages of using the
template parameter method, it's probably the right C++ idiom to use,
even if I don't like it much.  We have the gdb::Requires util, which
makes it at least more straightforward to read the code:

    template<typename T, typename V, typename = gdb::Requires<std::is_reference<T>>>

I'll push it with that change.  Should I have your RB?

Simon