fix c++17 compilation

include/unifex/await_transform.hpp

@@ -115,7 +115,7 @@ struct _awaitable_base<Promise, Value>::type {
     }
 
     template (class Error)
-      (requires exception_ptr_convertible<Error>)
+      (requires is_exception_ptr_convertible_v<Error>)
     void set_error(Error &&error) && noexcept {
       unifex::activate_union_member(result_->exception_, get_exception_ptr(std::forward<Error>(error)));
       result_->state_ = _state::exception;

include/unifex/get_exception_ptr.hpp

@@ -16,10 +16,9 @@
 #pragma once
 
 #include <unifex/tag_invoke.hpp>
+#include <unifex/type_traits.hpp>
 
-#include <concepts>
 #include <memory>
-#include <system_error>
 
 #include <unifex/detail/prologue.hpp>
 
@@ -33,11 +32,9 @@ namespace unifex
     }
   } get_exception_ptr{};
 
-  template <typename Error>
-  concept exception_ptr_convertible = requires(Error error) {
-    { get_exception_ptr(std::move(error)) } -> std::same_as<std::exception_ptr>;
-  };
-
+  template <typename T>
+  constexpr bool is_exception_ptr_convertible_v =
+      is_callable_v<_get_exception_ptr_cpo, remove_cvref_t<T>>;
 }  // namespace unifex
 
 #include <unifex/detail/epilogue.hpp>

test/get_exception_ptr_test.cpp

@@ -28,7 +28,7 @@ namespace unifex
   }
 }  // namespace unifex
 
-static_assert(exception_ptr_convertible<std::error_code>);
+static_assert(is_exception_ptr_convertible_v<std::error_code>);
 
 TEST(get_exception_ptr, error_code) {
   try {
@@ -49,7 +49,7 @@ std::exception_ptr tag_invoke(tag_t<get_exception_ptr>, test_error&& error) {
       std::runtime_error(std::to_string(error.error_code)));
 }
 
-static_assert(exception_ptr_convertible<test_error>);
+static_assert(is_exception_ptr_convertible_v<test_error>);
 
 TEST(get_exception_ptr, custom_error) {
   try {