Index: flang/runtime/environment.h
===================================================================
--- flang/runtime/environment.h
+++ flang/runtime/environment.h
@@ -9,6 +9,8 @@
 #ifndef FORTRAN_RUNTIME_ENVIRONMENT_H_
 #define FORTRAN_RUNTIME_ENVIRONMENT_H_
 
+#include "cpp-type.h"
+#include "entry-names.h"
 #include "flang/Decimal/decimal.h"
 #include <optional>
 
@@ -38,6 +40,14 @@
   Convert conversion;
 };
 extern ExecutionEnvironment executionEnvironment;
+
+extern "C" {
+// 16.9.51 COMMAND_ARGUMENT_COUNT
+//
+// Lowering may need to cast the result to match the precision of the default
+// integer kind.
+CppTypeFor<TypeCategory::Integer, 8> RTNAME(ArgumentCount)();
+}
 } // namespace Fortran::runtime
 
 #endif // FORTRAN_RUNTIME_ENVIRONMENT_H_