Index: flang/lib/Semantics/check-omp-structure.cpp
===================================================================
--- flang/lib/Semantics/check-omp-structure.cpp
+++ flang/lib/Semantics/check-omp-structure.cpp
@@ -1772,7 +1772,6 @@
 CHECK_SIMPLE_CLAUSE(Affinity, OMPC_affinity)
 CHECK_SIMPLE_CLAUSE(Allocate, OMPC_allocate)
 CHECK_SIMPLE_CLAUSE(Capture, OMPC_capture)
-CHECK_SIMPLE_CLAUSE(Copyin, OMPC_copyin)
 CHECK_SIMPLE_CLAUSE(Default, OMPC_default)
 CHECK_SIMPLE_CLAUSE(Depobj, OMPC_depobj)
 CHECK_SIMPLE_CLAUSE(Destroy, OMPC_destroy)
@@ -2381,6 +2380,27 @@
       currSymbols, dirClauseTriple, GetClauseKindForParserClass(x));
 }
 
+void OmpStructureChecker::Enter(const parser::OmpClause::Copyin &x) {
+  CheckAllowed(llvm::omp::Clause::OMPC_copyin);
+  for (const auto &ompObject : x.v.v) {
+    common::visit(
+        common::visitors{
+            [&](const parser::Designator &) {
+              if (const auto *name{parser::Unwrap<parser::Name>(ompObject)}) {
+                if (IsPolymorphicAllocatable(*name->symbol)) {
+                  context_.Say(name->source,
+                      "If a polymorphic variable with allocatable attribute "
+                      "'%s' is in COPYIN clause, the behavior is "
+                      "unspecified"_port_en_US, name->symbol->name());
+                }
+              }
+            },
+            [&](const parser::Name &) {}, // common block
+        },
+        ompObject.u);
+  }
+}
+
 llvm::StringRef OmpStructureChecker::getClauseName(llvm::omp::Clause clause) {
   return llvm::omp::getOpenMPClauseName(clause);
 }
Index: flang/test/Semantics/OpenMP/omp-copyin06.f90
===================================================================
--- /dev/null
+++ flang/test/Semantics/OpenMP/omp-copyin06.f90
@@ -0,0 +1,17 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp
+! OpenMP Version 5.0
+! 2.19.6.1 copyin Clause
+! If the list item is a polymorphic variable with the allocatable attribute,
+! the behavior is unspecified.
+
+subroutine copyin()
+  class(*), allocatable, save :: x
+  !$omp threadprivate(x)
+
+  !WARNING: If a polymorphic variable with allocatable attribute 'x' is in COPYIN clause, the behavior is unspecified
+  !$omp parallel copyin(x)
+    call sub()
+  !$omp end parallel
+
+end
+