Extended constructibility for co_error

Summary:
[Folly] Extended constructibility for `co_error` - constructible from everything `exception_wrapper` is constructible from, but not default-constructible. This permits:

```lang=c++
co_yield folly::coro::co_error(std::runtime_error("foo"));
```

Reviewed By: lewissbaker

Differential Revision: D19172210

fbshipit-source-id: df21c4f2a15ad2286e7cb34aae7c1be05e797a94

folly/experimental/coro/Error.h

@@ -16,6 +16,8 @@
 
 #pragma once
 
+#include <type_traits>
+
 #include <folly/ExceptionWrapper.h>
 
 namespace folly {
@@ -23,7 +25,14 @@ namespace coro {
 
 class co_error {
  public:
-  explicit co_error(exception_wrapper ex) : ex_(std::move(ex)) {}
+  template <
+      typename... A,
+      std::enable_if_t<
+          sizeof...(A) && std::is_constructible<exception_wrapper, A...>::value,
+          int> = 0>
+  explicit co_error(A&&... a) noexcept(
+      std::is_nothrow_constructible<exception_wrapper, A...>::value)
+      : ex_(static_cast<A&&>(a)...) {}
 
   const exception_wrapper& exception() const {
     return ex_;

folly/experimental/coro/test/ErrorTest.cpp

+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/Portability.h>
+
+#if FOLLY_HAS_COROUTINES
+
+#include <folly/experimental/coro/Error.h>
+
+#include <type_traits>
+
+#include <folly/ExceptionWrapper.h>
+#include <folly/Utility.h>
+#include <folly/portability/GTest.h>
+
+class CoErrorTest : public testing::Test {};
+
+TEST_F(CoErrorTest, constructible) {
+  using namespace folly;
+  using namespace folly::coro;
+
+  EXPECT_TRUE((std::is_constructible_v<co_error, exception_wrapper>));
+  EXPECT_TRUE((std::is_constructible_v<co_error, std::runtime_error>));
+  EXPECT_TRUE((std::is_constructible_v<
+               co_error,
+               in_place_type_t<std::runtime_error>,
+               std::string>));
+  EXPECT_FALSE((std::is_constructible_v<co_error, int>));
+}
+
+#endif