[flang][OpenMP][Semantics] Don't allow reduction of derived type components (#125480)

Before this patch, reduction of derived type components crashed the
compiler when trying to create the omp.declare_reduction.

In OpenMP 3.1 the standard says "a list item that appears in a reduction
clause must be a named variable of intrinsic type" (page 106). As I
understand it, a derived type component is not a variable.

OpenMP 4.0 added declare reduction, partly so that users could define
their own reductions on derived types. The above wording was removed
from the standard but derived type components were never explicitly
allowed.

OpenMP 5.0 added "A variable that is part of another variable, with the
exception of array elements, cannot appear in17 a reduction clause".

All standard versions also require the reduction argument to be
"definable", which roughly means that it is a variable. A
derived type component is more like an expression.

Fixes #125445

Author: tblah

flang/lib/Semantics/check-omp-structure.cpp

@@ -3225,9 +3225,12 @@ void OmpStructureChecker::CheckReductionObjects(
     }
   }
 
+  // Denied in all current versions of the standard because structure components
+  // are not definable (i.e. they are expressions not variables).
+  // Object cannot be a part of another object (except array elements).
+  CheckStructureComponent(objects, clauseId);
+
   if (version >= 50) {
-    // Object cannot be a part of another object (except array elements)
-    CheckStructureComponent(objects, clauseId);
     // If object is an array section or element, the base expression must be
     // a language identifier.
     for (const parser::OmpObject &object : objects.v) {

flang/test/Semantics/OpenMP/reduction-derived-component.f90

@@ -0,0 +1,10 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
+subroutine intrinsic_reduction
+  type local
+     integer alpha
+  end type local
+  type(local) a
+!ERROR: A variable that is part of another variable cannot appear on the REDUCTION clause
+!$omp parallel reduction(+:a%alpha)
+!$omp end parallel
+end subroutine intrinsic_reduction