git: 450f3e55bdad - stable/13 - Work around bogus old gcc "initializer element is not constant" error

[Mark Millard]

On 2021-Jun-26, at 06:36, Dimitry Andric <dim at FreeBSD.org> wrote:

> On 26 Jun 2021, at 06:11, Mark Millard <marklmi at yahoo.com> wrote:
>> 
>> Dimitry Andric dim at FreeBSD.org wrote on
>> Fri Jun 25 18:46:00 UTC 2021 :
>> 
>> . . .
>>>  In file included from /workspace/src/lib/msun/src/s_llround.c:11:0:
>>>   /workspace/src/lib/msun/src/s_lround.c:54:31: error: initializer element is not constant
>>>    static const type dtype_min = type_min - 0.5;
>>>                                  ^~~~~~~~
>>>   /workspace/src/lib/msun/src/s_lround.c:55:31: error: initializer element is not constant
>>>    static const type dtype_max = type_max + 0.5;
>>>                                  ^~~~~~~~
>>> 
>>>   Since 'type_min' and 'type_max' are constants declared just above these
>>>   lines this error is nonsensical, but older gcc's are not smart enough.
>> . . .
>> 
>> Well, in C "const" historically means closer to "read-only" than
>> to is-a-constant-expression in the language, unfortunately. Part
>> of this is the conversion away from being an lvalue (so: where
>> an lvalue is not required) loses the const qualification as part
>> of the conversion.
> 
> FWIW, this changed in gcc 8.1+, here:
> https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=f9c59f7e9511856bd6dc13d2d4904ebd9249c095
> 
> referencing these bugs:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66618
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69960
> 
> In one of the comments, joseph at codesourcery.com mentions:
>> Although diagnosing this probably makes sense, it's not required by the
>> standard ("An implementation may accept other forms of constant
>> expressions." - and this expression doesn't contain "assignment,
>> increment, decrement, function-call, or comma operators", so isn't
>> required by the Constraints for constant expressions not to be one).
> 
> I guess the gcc people also decided that POLA applied here. :)

Yea. If one wants one's C code to reliably compile in valid
C compilers generally, one must avoid treating such things
as type_min in:

static const type type_min = (type)DTYPE_MIN;

as any of an integer, floating, enumeration, or character
constant (in C terms). (C++ used the likes of "integer
literal" as terminology to avoid the ambiguity that makes
"integer constant" read funny vs. what is a constant
expression.)

Otherwise some compilers may reject the code without
violating the language standard.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)