add second example of broken optional::take_if

libsafecxx/test/broken/optional_take_if2.cxx

@@ -0,0 +1,71 @@
+#feature on safety
+
+template<class T+>
+T replace/(a)(T^/a dst, T src) safe
+{
+  unsafe {
+    T result = __rel_read(addr *dst);
+    __rel_write(addr *dst, rel src);
+    return result;
+  }
+}
+
+template<class F, class R, class ...Args>
+concept FnMut = requires(F f, Args ...args)
+{
+  requires safe(mut f(^args...));
+};
+
+template<class T+>
+choice optional
+{
+  default none,
+  [[safety::unwrap]] some(T);
+
+  T unwrap(self) noexcept safe {
+    return match(rel self) -> T {
+      .some(t) => rel t;
+      .none    => throw "invalid";
+    };
+  }
+
+  template<class P>
+  optional<T> take_if(self^, P p) safe
+  requires FnMut<P, bool, T>
+  {
+    return match(*self) -> optional<T> {
+      .some(^x) => return (
+        p(x) ? replace<optional<T>>(self, .none) : optional<T>::none
+      );
+      .none => return .none;
+    };
+  }
+
+  bool is_some(self const^) noexcept safe {
+    return match(*self) {
+      .some(_) => true;
+      .none => false;
+    };
+  }
+
+  bool is_none(self const^) noexcept safe {
+    return !self.is_some();
+  }
+};
+
+int main() safe
+{
+  struct C
+  {
+    static
+    bool invoke/(a)(int^/a x) safe {
+      bool b = (*x < 4321);
+      return b;
+    }
+  };
+
+  optional<int> opt = .some(1234);
+  auto m_p = mut opt.take_if(addr C::invoke);
+
+  if(!m_p.is_some()) throw 1234;
+}