https://bugs.gentoo.org/687450

project.h:210:14: error: friend declaration of ‘bool ReadGPR_OLD(project&, std::istream&, bool, bool)’ specifies default arguments and isn’t a definition [-fpermissive]
  210 |  friend bool ReadGPR_OLD(project &, istream &, bool, bool = false);  ///< this is for the very old version.
      |              ^~~~~~~~~~~

--- a/src/project.h
+++ b/src/project.h
@@ -122,6 +122,12 @@
 	graphical user interface.
 */
 
+class project;
+bool ReadGPR_OLD(project &, istream &, bool, bool = false);
+bool ReadGPR_v100(project &, istream &, bool, bool = false);
+bool ReadGPR_v110(project &, istream &, bool, bool = false);
+bool ReadGPR(project &, istream &, bool, bool = false);
+
 class project :
 	public custom_transformer_client,
 	public model
@@ -207,11 +213,11 @@
 
 // methods for file I/O : ReadGPR and WriteGPR are friend functions so that it would be easier to "borrow" them elsewhere...
 // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-	friend bool ReadGPR_OLD(project &, istream &, bool, bool = false);		///< this is for the very old version.
-	friend bool ReadGPR_v100(project &, istream &, bool, bool = false);		///< this is for the version 1.00.
-	friend bool ReadGPR_v110(project &, istream &, bool, bool = false);		///< this is for the version 1.10.
+	friend bool ReadGPR_OLD(project &, istream &, bool, bool);		///< this is for the very old version.
+	friend bool ReadGPR_v100(project &, istream &, bool, bool);		///< this is for the version 1.00.
+	friend bool ReadGPR_v110(project &, istream &, bool, bool);		///< this is for the version 1.10.
 /// This is an input function for the v1.11 ghemical file format.
-	friend bool ReadGPR(project &, istream &, bool, bool = false);
+	friend bool ReadGPR(project &, istream &, bool, bool);
 	
 	friend void WriteGPR_v100(project &, ostream &);	///< this is for the version 1.00.
 /// This is an output function for the v1.11 ghemical file format.