fix(execution): work around MSVC constraint evaluation bug in header mode (#228)

Author: ecoezen

include/beman/execution/detail/completion_signatures_for.hpp

@@ -32,15 +32,25 @@ namespace beman::execution::detail {
 struct no_completion_signatures_defined_in_sender {};
 
 /*!
- * \brief Primary template declaration for the customization of sender completion signatures.
+ * \brief Concept to check if an execution tag has a valid get_completion_signatures member template.
+ * \note  Extracted into a concept to work around an MSVC bug that occurs only header builds where
+ *        'requires' expressions containing dependent types incorrectly evaluate to false when placed
+ *        directly inside a function template body.
  * \headerfile beman/execution/execution.hpp <beman/execution/execution.hpp>
  * \internal
  */
+template <typename Tag, typename Sender, typename... Env>
+concept has_valid_complsig_specialization = requires { Tag::template get_completion_signatures<Sender, Env...>(); };
 
+/*!
+ * \brief Primary template declaration for the customization of sender completion signatures.
+ * \headerfile beman/execution/execution.hpp <beman/execution/execution.hpp>
+ * \internal
+ */
 template <typename Sender, typename... Env>
 consteval auto get_completion_signatures_for_helper() {
     using tag_t = ::std::remove_cvref_t<::beman::execution::tag_of_t<::std::remove_cvref_t<Sender>>>;
-    if constexpr (requires { tag_t::template get_completion_signatures<Sender, Env...>(); })
+    if constexpr (has_valid_complsig_specialization<tag_t, Sender, Env...>)
         return tag_t::template get_completion_signatures<Sender, Env...>();
     else
         return ::beman::execution::detail::no_completion_signatures_defined_in_sender{};