diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10679,9 +10679,12 @@
 def warn_omp_loop_64_bit_var : Warning<
   "OpenMP loop iteration variable cannot have more than 64 bits size and will be narrowed">,
   InGroup<OpenMPLoopForm>;
-def err_omp_unknown_reduction_identifier : Error<
+def err_omp_unknown_reduction_identifier_prior_omp_6_0 : Error<
   "incorrect reduction identifier, expected one of '+', '-', '*', '&', '|', '^', "
   "'&&', '||', 'min' or 'max' or declare reduction for type %0">;
+def err_omp_unknown_reduction_identifier_since_omp_6_0 : Error<
+  "incorrect reduction identifier, expected one of '+', '*', '&', '|', '^', "
+  "'&&', '||', 'min' or 'max' or declare reduction for type %0">;
 def err_omp_not_resolved_reduction_identifier : Error<
   "unable to resolve declare reduction construct for type %0">;
 def err_omp_reduction_ref_type_arg : Error<
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -19169,6 +19169,8 @@
   // operators: +, -, *, &, |, ^, && and ||
   switch (OOK) {
   case OO_Plus:
+    BOK = BO_Add;
+    break;
   case OO_Minus:
     // Minus(-) operator is not supported in TR11 (OpenMP 6.0). Setting BOK to
     // BO_Comma will automatically diagnose it for OpenMP > 52 as not allowed
@@ -19418,9 +19420,14 @@
     }
     if (BOK == BO_Comma && DeclareReductionRef.isUnset()) {
       // Not allowed reduction identifier is found.
-      S.Diag(ReductionId.getBeginLoc(),
-             diag::err_omp_unknown_reduction_identifier)
-          << Type << ReductionIdRange;
+      if (S.LangOpts.OpenMP > 52)
+        S.Diag(ReductionId.getBeginLoc(),
+               diag::err_omp_unknown_reduction_identifier_since_omp_6_0)
+            << Type << ReductionIdRange;
+      else
+        S.Diag(ReductionId.getBeginLoc(),
+               diag::err_omp_unknown_reduction_identifier_prior_omp_6_0)
+            << Type << ReductionIdRange;
       continue;
     }