Support set -add on flag types

Fixes #1082

src/enum.hh

@@ -73,6 +73,15 @@ EnableIfWithoutBitOps<Enum> option_from_string(StringView str, Enum& e)
     e = it->value;
 }
 
+template<typename Flags, typename = decltype(enum_desc(Flags{}))>
+EnableIfWithBitOps<Flags, bool> option_add(Flags& opt, StringView str)
+{
+    Flags res = Flags{};
+    option_from_string(str, res);
+    opt |= res;
+    return res != (Flags)0;
+}
+
 }
 
 #endif // enum_hh_INCLUDED

src/option_types.hh

@@ -214,8 +214,7 @@ inline bool option_add(StronglyTypedNumber<RealType, ValueType>& opt,
     return val != 0;
 }
 
-template<typename T>
-bool option_add(T&, StringView str)
+inline bool option_add(...)
 {
     throw runtime_error("no add operation supported for this option type");
 }