build: workaround gcc synthesis constructor cast warnings

cmdopt.hpp

@@ -115,7 +115,7 @@ namespace util { namespace cmdopt {
 
         class bytes : public value<size_t> {
         public:
-            using value<size_t>::value;
+            bytes (size_t &_value): value (_value) { }
 
             using value<size_t>::execute;
             void execute (const char *restrict) override;

json/except.hpp

@@ -24,13 +24,17 @@
 namespace json {
     /// The base class for all exceptions throw directly by the json namespace.
     struct error : public std::runtime_error {
-        using runtime_error::runtime_error;
+        error (const std::string &what):
+            runtime_error (what)
+        { ; }
     };
 
 
     /// Base class for all type conversion errors
     struct type_error : public error {
-        using error::error;
+        type_error (const std::string &what):
+            error (what)
+        { ; }
     };
 
 
@@ -47,7 +51,9 @@ namespace json {
 
     /// Base class for errors thrown during schema validation
     struct schema_error : public error {
-        using error::error;
+        schema_error (const std::string &what):
+            error (what)
+        { ; }
     };
 
 

json/schema.cpp

@@ -28,13 +28,17 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 struct length_error : public json::schema_error {
-    using schema_error::schema_error;
+    length_error (const std::string &what):
+        schema_error (what)
+    { ; }
 };
 
 
 //-----------------------------------------------------------------------------
 struct format_error : public json::schema_error {
-    using schema_error::schema_error;
+    format_error (const std::string &what):
+        schema_error (what)
+    { ; }
 };