[RFC] libstdc++/6720 and libstdc++/6671

[Mark Mitchell]

--On Wednesday, May 22, 2002 10:46:03 PM +0100 Neil Booth 
<neil@daikokuya.demon.co.uk> wrote:

> Gabriel Dos Reis wrote:-
>
>> The standard does specify the effect of #include <cmath> in a program
>> (with on exception, see below): it should make the declarations given
>> in section 26 available to the program.
>> The specification of all standard headers is given in 17.4.1.2.
>>
>> The exception is 17.4.3.2:
>>
>>    *If* a file with a name equivalent to the derived file name for one of
>>    the C++ Standard Library headers is *not provided* as part of the
>>    implementation, *and* a file with that name is placed in any of the
>>    standard places for a source file to be included (16.2), the behavior
>>    is undefined.
>>
>> (Emphasis is mine).  Therefore, the contexts in which the compiler is
>> granted right to transmute semantics is very limited:
>>
>>    1) the implementation _doesn't_ provide a file derived from a
>>       standard header as part of its implementation. *and*
>>
>>    2) a file with a name derived from a standard header should be
>>       placed in any of the standard places for a source file to be
>>       included.
>>
>> Since we're not doing 1) (i.e. we implement standard headers in terms
>> of actual files), we're not in case where we could legitimately invoke
>> undefined behaviour; that is we have to deliver the semantics
>> according to 17.4. and 16.2 -- we must find the "right" <cmath>.
>
> Sure.  But how is this different from any other switch you might
> provide G++ like -I. or -ffast-math?

I'd be interested to hear what other committee members thing the
language that Gaby quoted means.  I can certainly read it to mean what
Gaby thought it meant; in particular, that even if -I options are in
play you get the right header.  You can read it that way due to the
fact that all of 16.2 is essentially implementation-defined; the
paragraph Gaby's quoting seems to say "even despite all that
implementation-defined stuff, this particular usage has defined
behavior."

I don't know *why* the standard would say that.  If it were up to me,
I'd certainly rather just let -I do whatever it normally does.  But,
we have to work with what we've got.

Do other people reading this have insight into what the committee
really meant here?

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com