[Bug middle-end/106504] New: [OpenMP] 'for simd linear(i:1)' should be rejected with 'parallel private(i)'

[burnus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106504

            Bug ID: 106504
           Summary: [OpenMP] 'for simd linear(i:1)' should be rejected
                    with 'parallel private(i)'
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, diagnostic, openmp
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burnus at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

The following program is a bit odd – and prints at runtime 10 for one thread –
and 6 for all others.

And I believe it is invalid for the reason given below:

Pre-remark: the 'for simd' directive has a predetermined 'linear(i:1)',
which can also be explicitly be specified without affecting the program.

------------------------------------
#include <stdio.h>
int main()
{
 int i;
 #pragma omp parallel private (i)
 {
   i = 6;
   #pragma omp for simd
   for (i = 1; i < 10; i++)
     ;
   printf ("%i\n", i);
 }
 return 0;
}
----------------------------------

I think the following applies:

"17.2 Clauses on Combined and Composite Constructs"

"If a list item of the linear clause is the iteration variable of a *simd* or
worksharing-loop SIMD construct and it is not declared in the construct, the
effect on the outer leaf constructs is as if the list item was specified in a
*lastprivate* clause on the combined or composite construct with the rules
specified above applied." 

And:
"Restrictions to the *lastprivate* clause are as follows:
 * A list item must not appear in a *lastprivate* clause on a work-distribution
   construct if the corresponding region binds to the region of a
   parallelism-generating construct in which the list item is private."