Fix build on gcc-8 & 9.

See https://lists.gnucash.org/pipermail/gnucash-devel/2023-March/046617.html

bindings/guile/gnc-optiondb.i

@@ -1192,9 +1192,9 @@ inline SCM return_scm_value(ValueType value)
 
     SCM save_scm_value()
     {
-        static const SCM plain_format_str{scm_from_utf8_string("~s")};
-        static const SCM ticked_format_str{scm_from_utf8_string("'~a")};
-        static const SCM list_format_str{scm_from_utf8_string("'~s")};
+        [[maybe_unused]] static const SCM plain_format_str{scm_from_utf8_string("~s")};
+        [[maybe_unused]] static const SCM ticked_format_str{scm_from_utf8_string("'~a")};
+        [[maybe_unused]] static const SCM list_format_str{scm_from_utf8_string("'~s")};
 //scm_simple_format needs a scheme list of arguments to match the format
 //placeholders.
         return std::visit([$self] (auto &option) -> SCM {
@@ -1460,7 +1460,7 @@ inline SCM return_scm_value(ValueType value)
                         if (scm_is_string(new_value))
                         {
                             auto strval{scm_to_utf8_string(new_value)};
-                            GncGUID guid;
+                            GncGUID guid{};
                             string_to_guid(strval, &guid);
                             auto book{get_current_book()};
                             option.set_value(xaccAccountLookup(&guid, book));
@@ -1573,7 +1573,7 @@ inline SCM return_scm_value(ValueType value)
                         if (scm_is_string(new_value))
                         {
                             auto strval{scm_to_utf8_string(new_value)};
-                            GncGUID guid;
+                            GncGUID guid{};
                             string_to_guid(strval, &guid);
                             auto book{get_current_book()};
                             option.set_default_value(xaccAccountLookup(&guid, book));

libgnucash/engine/gnc-option-impl.hpp

@@ -328,12 +328,12 @@ template<class OptType,
                                      is_RangeValue_v<OptType>), int> = 0>
 std::istream& operator>>(std::istream& iss, OptType& opt)
 {
-    std::decay_t<decltype(opt.get_value())> value;
     if constexpr (std::is_same_v<std::decay_t<decltype(opt.get_value())>, const _gncOwner*> ||
                   std::is_same_v<std::decay_t<decltype(opt.get_value())>, const _QofQuery*>)
         return iss;
     else
     {
+        std::decay_t<decltype(opt.get_value())> value;
         iss >> value;
         opt.set_value(value);
         return iss;