Why do we require “requires requires”?

Question

One of the corners of C++20 constraints is that there are certain situations in which you have to write requires requires. For instance, this example from [expr.prim.req]/3:

A requires-expression can also be used in a requires-clause ([temp]) as a way of writing ad hoc constraints on template arguments such as the one below:

template<typename T>
  requires requires (T x) { x + x; }
    T add(T a, T b) { return a + b; }

The first requires introduces the requires-clause, and the second introduces the requires-expression.

What is the technical reason behind needing that second requires keyword? Why can't we just allow writing:

template<typename T>
  requires (T x) { x + x; }
    T add(T a, T b) { return a + b; }

_{(Note: please don't answer that the grammar requires it)}

Answer 1

It is because the grammar requires it. It does.

A requires constraint does not have to use a requires expression. It can use any more-or-less arbitrary boolean constant expression. Therefore, requires (foo) must be a legitimate requires constraint.

A requires expression (that thing that tests whether certain things follow certain constraints) is a distinct construct; it's just introduced by the same keyword. requires (foo f) would be the beginning of a valid requires expression.

What you want is that if you use requires in a place that accepts constraints, you should be able to make a "constraint+expression" out of the requires clause.

So here's the question: if you put requires (foo) into a place that is appropriate for a requires constraint... how far does the parser have to go before it can realize that this is a requires constraint rather than a constraint+expression the way you want it to be?

Consider this:

void bar() requires (foo)
{
  //stuff
}

If foo is a type, then (foo) is a parameter list of a requires expression, and everything in the {} is not the body of the function but the body of that requires expression. Otherwise, foo is an expression in a requires clause.

Well, you could say that the compiler should just figure out what foo is first. But C++ really doesn't like it when the basic act of parsing a sequence of tokens requires that the compiler figure out what those identifiers mean before it can make sense of the tokens. Yes, C++ is context-sensitive, so this does happen. But the committee prefers to avoid it where possible.

So yes, it's grammar.