In any case, why is (2) better than (3)? What was the rationale for such a syntax in Elements of Programming? (At the end I can adopt whatever syntax but I will need to justify the choice in a "rationale" section.)
template< typename T > concept EqualityComparable = ... // (1) no good because concepts are not templates
concept EqualityComparable< typename T > = ... // (2)
concept <typename T> EqualityComparable = ... // (3)
I really can't say where the concept syntax in EoP came from, but the notation is traditionally mathematical.
If you buy into the idea that a concept is a function on template arguments, then I think that 2 fits with existing C++ syntax better than 3. After all, we don't write the type of functions ahead of their declarator. That is we don't declare min() as "T (T, T) min", where "T (T, T)" is the function type of min.
I think this is a reasonable rationale for (2) over (3) and (1). Unless someone can point out a reason against it, I will adopt (2) with the given rationale.
I tend to prefer 2. It seems more consistent with existing syntax.
I find following the notation of EoP a sufficient rationale itself. You are choosing the syntax that by now many people in the industry is familiar with. Regards, &rzej