Defined the StatusCode constant values explicitly.

This is needed to be able to use (for instance) constant references to these
constants in user code in C++14 mode. In C++17 mode the explicit declaration
is not needed anymore. But it also doesn't hurt the compilation.

GaudiKernel/src/Lib/StatusCode.cpp

@@ -11,6 +11,9 @@
 
 // statics
 bool StatusCode::s_checking( false );
+constexpr const StatusCode::ErrorCode StatusCode::SUCCESS;
+constexpr const StatusCode::ErrorCode StatusCode::FAILURE;
+constexpr const StatusCode::ErrorCode StatusCode::RECOVERABLE;
 
 namespace {
   /// Default StatusCode category

GaudiKernel/tests/src/test_StatusCode.cpp

@@ -407,3 +407,22 @@ BOOST_AUTO_TEST_CASE( conversions ) {
   // sc = 42;     // no implicit conversion from int
   // int i = sc;  // no implicit conversion to int
 }
+
+/// Function used for testing that the constants in @c StatusCode can be found
+/// by constant references.
+bool __attribute__( ( noinline ) )
+testReferencePassing( const StatusCode::ErrorCode& value,
+                      const StatusCode::ErrorCode& reference ) {
+  return ( value == reference );
+}
+
+BOOST_AUTO_TEST_CASE( const_definitions ) {
+  // Test whether the test can compile/link correctly when passing the
+  // constants in @c StatusCode into a function by constant reference.
+  BOOST_CHECK( testReferencePassing( StatusCode::SUCCESS,
+                                     StatusCode::SUCCESS ) );
+  BOOST_CHECK( testReferencePassing( StatusCode::FAILURE,
+                                     StatusCode::FAILURE ) );
+  BOOST_CHECK( testReferencePassing( StatusCode::RECOVERABLE,
+                                     StatusCode::RECOVERABLE ) );
+}