Why do we require requires requires?

by

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.

More Related Contents:

  1. Does constraint subsumption only apply to concepts?
  2. C++ Concept that requires a member function with an OutputIterator as parameter
  3. In which access control context are concepts evaluated?
  4. Is lambdification of a concept an improvement or bad practice?
  5. What is a “span” and when should I use one?
  6. Can I take the address of a function defined in standard library?
  7. What is the (“spaceship”, three-way comparison) operator in C++?
  8. What exactly are C++ modules?
  9. Why does same_as concept check type equality twice?
  10. Will specialization of function templates in std for program-defined types no longer be allowed in C++20?
  11. Can class template constructors have a redundant template parameter list in c++20
  12. Class type non-type template parameter initialization does not compile
  13. void_t “can implement concepts”?
  14. non-defaulted operator doesn’t generate == and != in C++20
  15. Designated initializers in C++20
  16. How to use c++20 modules with CMake?
  17. Which headers in the C++ standard library are guaranteed to include another header?
  18. What is `constinit` in C++20?
  19. Will consteval functions allow template parameters dependent on function arguments?
  20. How to use std::views::transform on a vector?
  21. Why is volatile deprecated in C++20?
  22. Why default three-way operator (spaceship ) generates equality operator (==) and user define three-way operator not?
  23. What is required for a custom BGL graph to work with topological sort?
  24. asio How to change the executor inside an awaitable?
  25. Why does Clang 12 refuse to initialize aggregates in the C++20 way?
  26. Why C++ ranges “transform -> filter” calls transform twice for values that match the filter’s predicate?
  27. Why must a std::ranges::filter_view object be non-const for querying its elements?
  28. Will consteval allow using static_assert on function arguments?
  29. if constexpr – why is discarded statement fully checked?
  30. if constexpr – why is discarded statement fully checked?

Leave a Comment