Why does `Parallel_tag` inherit from `Sequential_tag`?

[lrineau]

Why does Parallel_tag inherit from Sequential_tag? As far as I know Tag_true does not inherit from Tag_false, and that is for the best. Hopefully, t Why does Parallel_tag inherit from Sequential_tag? As far as I know Tag_ Why does Parallel_tag inherit from Sequential_tag? As far as I know Tag_true does not inherit from Tag_false, and that is for the best. Hopefully, that derivation is not documented, but I wonder if any code might break if we remove that derivation.true does not inherit from Tag_false, and that is for the best. Hopefully, that derivation is not documented, but I wonder if any code might break if we remove that derivation.hat derivation is not documented, but I wonder if any code might break if we remove that derivation.

[sloriot]

The only reason I can think of for the inheritance is if the tag is if an instance is used as a parameter of function that only has an overload with the sequential tag.

[mglisse]

My understanding is that a sequential implementation is a valid fallback when someone asks for a parallel implementation but none is available (like code for bidirectional iterators is a valid fallback when there is no implementation for random access iterators).
Now I don't have an opinion on whether it is a good idea. And the standard C++17 execution policies don't seem to use derivation or conversion, even if they do consider sequential as a valid fallback.