[Bug preprocessor/91517] New: Pragma expansion in variadic macro reorders pragmas and breaks code

[paboyle at ph dot ed.ac.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91517

            Bug ID: 91517
           Summary: Pragma expansion in variadic macro reorders pragmas
                    and breaks code
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: paboyle at ph dot ed.ac.uk
  Target Milestone: ---

Pragma expansion in variadic macro reorders pragmas and breaks code

Simple test code 

#define DO_PRAGMA(x) DO_PRAGMA_(x)
#define DO_PRAGMA_(x) _Pragma (#x)
#define thread_for( i, num, ... )  DO_PRAGMA(omp parallel for schedule(static))
for ( uint64_t i=0;i<num;i++) { __VA_ARGS__ } ;
#define thread_critical            DO_PRAGMA(omp critical);

thread_for(i,N,{
    int x=i;
    thread_critical {
      sum+=x;
    }
});


Expands incorrectly to:

Peters-Laptop:build peterboyle$ g++-9  -E tmp.cc  | uniq
# 1 "tmp.cc"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "tmp.cc"
#pragma omp critical
# 8 "tmp.cc"
#pragma omp parallel for schedule(static)
# 8 "tmp.cc"
 for ( uint64_t i=0;i<N;i++) { { int x=i; ; { sum+=x; } } } ;

With Critical pragma in WRONG place.

Clang++ and icpc produce right ordering:

# 1 "tmp.cc"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 374 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "tmp.cc" 2
#pragma omp parallel for schedule(static)
# 8 "tmp.cc"
 for ( uint64_t i=0;i<N;i++) { { int x=i;
# 8 "tmp.cc"
#pragma omp critical
# 8 "tmp.cc"
; { sum+=x; }} } ;;