requires-expression grammar trivia

A couple of footnotes to my previous post explaining requires requires.

First: I confidently defend the rationale behind noexcept(noexcept(...)) and requires requires(...), but that doesn’t mean I like everything about the latter syntax. I believe that requires-clauses are one of the best things about C++2a (at least, once someone sits down and figures out how they’re going to interact with name-mangling), but I believe requires-expressions are a complete garbage fire.

On that subject, here’s a puzzle that some of my readers will have seen before, on Slack or in my CppCon talk. This is its blog debut. Godbolt link:

template<class T>
concept Negatable = requires(T t) {
    -t -> T;

static_assert(Negatable<char>);  // FAILS -- WHY?

The solution to this puzzle will be presented below.

Second: Reddit commenter Ameisen writes:

I feel like if the requires clause were given an expression, it should just treat it as an expression to evaluate…

which echoes Barry’s original StackOverflow suggestion

Why can’t we just allow writing:

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

The problem with this idea is that C++ is unparseable! How do you know if you’ve been “given an expression” or not? Consider

template<class T> void f(T) requires requires(T (x)) { (void)x; };

With two requireses, the current C++2a working draft would parse this as a function declaration equivalent to

template<class T>
void f(T) requires (
    requires (T x) {
        { (void)x };

With one requires, it’d be parsed as a function definition equivalent to

template<class T>
void f(T) requires T(x)


(Here, both xs refer to some in-scope variable not shown in this snippet, such as constexpr int x = 42. So requires (T(x)) means “participates in overload resolution when x, explicitly converted to T, is truthy.” Compilers differ on exactly what happens when T(x) is falsey, ill-formed, or non-constant.)

This example relies on several unnecessary quirks of the C++ grammar:

  • A stray semicolon at file/namespace scope is simply ignored.

  • A function parameter can redundantly be enclosed in parentheses.

  • A requirement-seq can contain simple-requirements which lack the outer pair of curly braces (that is, requires(T x) { (void)x; } is treated as a valid requires-expression equivalent to requires(T x) { { (void)x }; }). This is crucial to my snippet because { (void)x; } is a valid function body, whereas { { (void)x }; } is not.

The third quirk above is a source of error for real programs. Did you figure out the Negatable puzzler at the top of this post? Every time I present that puzzle, the very first thing people say is, “Ooh, is this related to integer promotion?” — but no, it’s actually the missing curly braces!

Now, to be fair, Saar Raz’s Clang branch actually gives a decent error message if you try the static_assert exactly as written in this post:

error: static_assert failed
^             ~~~~~~~~~~~~~~~
note: because 'char' does not satisfy 'Negatable'
note: because '-t->T' would be invalid: member reference type 'char' is not a pointer
    -t -> T;

That’s why I massaged the Godbolt example to use a SFINAE context, wherein the compiler never feels any compunction to explain itself. (Or, for a similar effect, you can always use GCC. :))

Walter Brown and Casey Carter’s P1084 “Today’s return-type-requirements are Insufficient” (November 2018) describes a previous suggestion that the “fat arrow” => could be introduced as a new token, in cases where the normal arrow -> had had insufficient or incorrect behavior (because its behavior had been incorrect in most cases). P1084’s actual solution was to change the behavior of -> so that it was more frequently correct. However, I wonder if “fat arrow” should be introduced anyway, and mandated.

template<class T>
concept Negatable = requires(T t) {
    { -t } => T;    // unambiguous
    -t => T;        // unambiguous
    { -t } -> T;    // should be invalid: -> should never be the "convertible to" separator
    -t -> T;        // should be valid and unambiguous: -> should always be the arrow operator

My number one preference, though, is to see requires-expressions and concept definitions completely removed from C++2a — or a commitment to postpone the release of C++2a until concepts are ready for general use. Using => instead of -> would just be an incremental improvement: a small band-aid on top of the garbage fire.

Even if C++ got rid of the “parenthesized parameter” ambiguity, new ambiguities are being added to the language all the time. For example, Daveed Vandevoorde’s P0632 “Down with typename!” was recently adopted into the C++2a Working Draft. It introduces some new areas of ambiguity into the grammar (cynic says: basically as chaff to occupy some dev time for GCC and Clang while EDG catches up with implementing the more important parts of C++2a).

template<class T>
void f(T) requires requires(X<T>::type && x) { (void)x; };

This would be unambiguously ill-formed C++2a code without P0632. With P0632, X<T>::type would be treated as a type-expression because it happens to appear in a position where a “parameter type” is expected. So, in C++2a as it stands right now, P0632 having been adopted, this snippet would change meanings if one of the requireses were removed.

template<class T>
void f(T) requires requires(X<T>::type && x) { (void)x; };

is a function declaration with the constraint requires(typename X<T>::type&& x) { { (void)x }; }.

template<class T>
void f(T) requires(X<T>::type && x) { (void)x; };

is a function declaration with the constraint (X<T>::type && x) and the function body { (void)x; }.

…well, okay, P0632 actually doesn’t say that typename is optional in this particular context, but I’m pretty sure that’s a defect in the current draft (which I just reported to the Core Working Group earlier today). When we have so many disparate and fairly invasive features going into the same document from different directions, it’s easy for those features to fail to play together. And the Committee doesn’t give itself much “fudge time” to iron out these details, either — people are still confidently calling it “C++20” — as in 2020 — even as we enter the year 2019, with no vendor having completed an implementation of C++17 yet!

Posted 2019-01-17