[gcc/devel/omp/gcc-16] Fortran/OpenMP: Improve declare-reduction diagnostic

https://gcc.gnu.org/g:816931db9e1301f05ae9075bb367d5cda92ae6c8

commit 816931db9e1301f05ae9075bb367d5cda92ae6c8
Author: Tobias Burnus <[email protected]>
Date:   Tue Jun 9 13:58:03 2026 +0200

    Fortran/OpenMP: Improve declare-reduction diagnostic
    
    Change the diagnostic for the ambiguity check for 'omp declare reduction'
    to actually output the reduction operator/idenfier and the type to which
    this reduction applies to.
    
    gcc/fortran/ChangeLog:
    
            * module.cc (load_omp_udrs): Improve reduction diagnostic output.
    
    gcc/testsuite/ChangeLog:
    
            * gfortran.dg/gomp/declare-reduction-1.f90: New test.
    
    (cherry picked from commit fd268b83ffb58ae700380e1e52af279f782fdb38)

Diff:
---
 gcc/fortran/module.cc                              | 13 ++++----
 .../gfortran.dg/gomp/declare-reduction-1.f90       | 39 ++++++++++++++++++++++
 2 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/gcc/fortran/module.cc b/gcc/fortran/module.cc
index 04ddf6b4476b..38c9b295f6b9 100644
--- a/gcc/fortran/module.cc
+++ b/gcc/fortran/module.cc
@@ -76,6 +76,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "parse.h" /* FIXME */
 #include "constructor.h"
 #include "cpp.h"
+#include "diagnostic-core.h"
 #include "scanner.h"
 #include <zlib.h>
 
@@ -5443,12 +5444,12 @@ load_omp_udrs (void)
 	  pointer_info *p = get_integer (atom_int);
 	  if (strcmp (p->u.rsym.module, udr->omp_out->module))
 	    {
-	      gfc_error ("Ambiguous !$OMP DECLARE REDUCTION from "
-			 "module %s at %L",
-			 p->u.rsym.module, &gfc_current_locus);
-	      gfc_error ("Previous !$OMP DECLARE REDUCTION from module "
-			 "%s at %L",
-			 udr->omp_out->module, &udr->where);
+	      gfc_error ("Ambiguous !$OMP DECLARE REDUCTION %qs for type %qs "
+			 "from module %qs at %L", udr->name,
+			 gfc_typename (&ts), module_name, &gfc_current_locus);
+	      inform (gfc_get_location (&udr->where),
+		      "Previous !$OMP DECLARE REDUCTION from module %qs",
+		      udr->omp_out->module);
 	    }
 	  skip_list (1);
 	  continue;
diff --git a/gcc/testsuite/gfortran.dg/gomp/declare-reduction-1.f90 b/gcc/testsuite/gfortran.dg/gomp/declare-reduction-1.f90
new file mode 100644
index 000000000000..32f00aaf2b0a
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/gomp/declare-reduction-1.f90
@@ -0,0 +1,39 @@
+! { dg-do compile }
+
+module one
+  implicit none
+  type t
+    integer :: x
+  end type t
+end module
+
+module two
+  use one
+  implicit none
+  !$omp declare reduction(+ : t : omp_out%x = omp_out%x + omp_in%x) initializer (omp_priv%x = 0)
+end module
+
+module three
+  use one
+  implicit none
+  !$omp declare reduction(+ : t : omp_out%x = omp_out%x + omp_in%x) initializer (omp_priv%x = 0)
+end module
+
+subroutine sub2
+  use two    ! { dg-note "Previous !.OMP DECLARE REDUCTION from module 'two'" }
+  use three  ! { dg-error "Ambiguous !.OMP DECLARE REDUCTION 'operator \\+' for type 'TYPE\\(t\\)' from module 'three' at .1." }
+  implicit none
+  type(t) :: var(3), sum
+  integer :: i
+
+  var(:)%x = [1,2,3]
+  sum%x = 0
+
+  !$omp parallel do reduction(+: sum)
+  do i = 1, 3
+    sum%x = sum%x + var(i)%x 
+  end do
+end
+
+call sub2
+end