[flang][OpenMP] Don't allow namelist variables with threadprivate

While the standard doesn't explicitly mention namelist variables in this
case, it does prohibit privatization of namelist variables. Flang
decided to also prohibit namelist variables from appearing in a
reduction clause.

Variables that are part of a namelist are linked to I/O mechanisms, and
their memory association and initialization are managed by the compiler
in a way that conflicts with the requirements of threadprivate
variables. I propose we should add this restriction.

Author: tblah

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

@@ -1516,6 +1516,10 @@ void OmpStructureChecker::CheckThreadprivateOrDeclareTargetVar(
                       "A variable in a %s directive cannot appear in an "
                       "EQUIVALENCE statement"_err_en_US,
                       ContextDirectiveAsFortran());
+                } else if (name->symbol->test(Symbol::Flag::InNamelist)) {
+                  context_.Say(name->source,
+                      "A variable in a %s directive cannot appear in a NAMELIST"_err_en_US,
+                      ContextDirectiveAsFortran());
                 } else if (name->symbol->test(Symbol::Flag::OmpThreadprivate) &&
                     GetContext().directive ==
                         llvm::omp::Directive::OMPD_declare_target) {
@@ -1571,6 +1575,11 @@ void OmpStructureChecker::CheckThreadprivateOrDeclareTargetVar(
                           ContextDirectiveAsFortran(), obj->name(),
                           name.symbol->name());
                     }
+                    if (obj->test(Symbol::Flag::InNamelist)) {
+                      context_.Say(name.source,
+                          "A variable in a %s directive cannot appear in a NAMELIST"_err_en_US,
+                          ContextDirectiveAsFortran());
+                    }
                   }
                 }
               }

flang/test/Semantics/OpenMP/threadprivate09.f90

@@ -0,0 +1,12 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 %openmp_flags
+
+! Check that namelist variables cannot be used with threadprivate
+
+module m
+  integer :: nam1
+  common /com1/nam1
+  namelist /nam/nam1
+
+  !ERROR: A variable in a THREADPRIVATE directive cannot appear in a NAMELIST
+  !$omp threadprivate(/com1/)
+end