However, syntactically you use it similarly to a template:
MyConcept<T1, T2, T3>
my_template<T1, T2, T3>
Syntactically yes, but they are allowed to mean very different things.
concept <typename T> EqualityComparable = ... // (3)
Actually, this would make the most sense to me, why wouldn't we use this syntax (3)?
Alex wanted the syntax to reflect, to some extent, the style that he used to define concepts in Elements of Programming. A concept is a type predicate (function) defined by a conjunction of syntactic and semantic requirements. Compare: // EoP (with pseudo-C++ syntax) Relation(Op) = Predicate(Op) && HomogeneousFunction(Op) && Arity(Op) == 2 // N3351 concept Relation<typename Op, typename T> = Predicate<Op, T, T>; In the writing, we wanted to emphasize the idea that a concept was "just" a predicate on template arguments. That made it easier to focus on the algorithm's requirements rather than to figure out the extensive set of language rules that would make it work.