--- slepc_spectral_transformation.h	2020-05-21 00:41:03.000000000 +0300
+++ slepc_spectral_transformation.h	2020-06-21 12:16:33.800000000 +0300
@@ -44,6 +44,8 @@
 
 namespace SLEPcWrappers
 {
+	// forward declaration
+	class SolverBase;
   /**
    * Base class for spectral transformation classes using the SLEPc solvers
    * which are selected based on flags passed to the spectral transformation.