feat: recognize requires expressions

Author: JohelEGP

regression-tests/pure2-concept-definition-no-pitfall-1.cpp2

@@ -0,0 +1,7 @@
+// https://youtu.be/CXn02MPkn8Y?t=2337
+negatable: <T> concept = requires(t: T)
+{
+  _ = -t is T; // Hopefully obviously wrong. Should be `{ -t } is T;`.
+};
+
+main: () = static_assert(negatable<char>);

regression-tests/pure2-concept-definition-no-pitfall-2.cpp2

@@ -0,0 +1,8 @@
+// https://youtu.be/CXn02MPkn8Y?t=2418
+int_sized: <T> concept = requires(t: T)
+{
+  _ = sizeof(T) == 4; // Hopefully obviously wrong. Should be `requires (sizeof(T) == 4);`.
+};
+// Could also be `int_sized: <T> concept = sizeof(T) == 4;`.
+
+main: () = static_assert(int_sized<char>);

regression-tests/pure2-concept-definition-no-pitfall-3.cpp2

@@ -0,0 +1,7 @@
+// https://youtu.be/CXn02MPkn8Y?t=2455
+nothrow_incrementable: <T> concept = requires(inout t: T)
+{
+  _ = noexcept(t++); // Hopefully obviously wrong. Should be `requires noexcept(t++);` or `{ t++ } !throws;`.
+};
+
+main: () = { static_assert(nothrow_incrementable<char>); }

regression-tests/pure2-concept-definition-pit-of-success-1.cpp2

@@ -0,0 +1,22 @@
+// https://quuxplusone.github.io/blog/2021/06/09/another-concepts-chest-mimic/
+has_a_but_not_b: <T> concept = requires(t: T)
+{
+  _           = a(t);
+  !requires _ = b(t); // In Cpp2, this works and does the correct thing.
+};
+
+s1: @struct type = { }
+s2: @struct type = { }
+a: (_: s2)       = { }
+s3: @struct type = { }
+b: (_: s3)       = { }
+s4: @struct type = { }
+a: (_: s4)       = { }
+b: (_: s4)       = { }
+
+main: () = {
+  static_assert(!has_a_but_not_b<s1>); // as expected
+  static_assert(has_a_but_not_b<s2>);  // as expected
+  static_assert(!has_a_but_not_b<s3>); // as expected
+  static_assert(!has_a_but_not_b<s4>); // pit of success!
+}

regression-tests/pure2-concept-definition.cpp2

@@ -1,5 +1,26 @@
 arithmetic: <T> concept = std::integral<T> || std::floating_point<T>;
-main: ()                = {
-  assert<Testing>( arithmetic<i32> );
-  assert<Testing>( arithmetic<float> );
+number_difference_t: <T> type == std::type_identity_t<decltype(T() - T())>;
+number: <T> concept = std::regular<T> && requires(c: T)
+{
+  !requires std::iter_reference_t<T>;                   // Negative requirement.
+  {c + c} is std::common_with<T>;                       // Compound requirement.
+  number_difference_t<T>;                               // Type requirement.
+  _ = c - c;                                            // Expression requirement.
+  requires std::common_with<number_difference_t<T>, T>; // Nested requirement.
+};
+test_nonthrowing_requirements: <T> concept = requires
+{ // clang-format off
+  { T() } !throws;
+  { -T() } !throws, is std::same_as<T>;
+}; // clang-format on
+main: () = {
+  static_assert(arithmetic<i32>);
+  static_assert(arithmetic<float>);
+  static_assert(number<i32>);
+  static_assert(number<float>);
+  static_assert(number<std::chrono::seconds>);
+  static_assert(!number<* i32>);
+  static_assert(!number<std::reverse_iterator<* i32>>);
+  static_assert(test_nonthrowing_requirements<i32>);
+  static_assert(!test_nonthrowing_requirements<std::chrono::seconds>);
 }

regression-tests/pure2-print.cpp2

@@ -84,6 +84,19 @@ outer: @print type = {
             is 43 = "forty-and-three";
             is _  = "default case";
         } << "\n";
+
+        _ =
+          requires { std::vector<int>; }
+          + requires { _ = 0; }
+          + requires {
+              requires true;
+              !requires std::vector<int>;
+              !requires _ = 0;
+              { 0 };
+              { 0 } !throws;
+              { 0 } is std::regular;
+              { 0 } !throws, is std::regular;
+            };
     }
 
     x: <Ts...: type> type = {

regression-tests/pure2-requires-clauses.cpp2

@@ -17,6 +17,9 @@ f:  <T: type, U: type>
 
 v: <T> const T requires std::same_as<T, i32> = 0;
 
+g: <T> () requires true = { }
+g: () (requires { _ = 0; });
+
 main: () = {
     _: X<int,int> = ();
     std::cout << f<int,int>(2,5);