added substitute_for concept

include/eve/concept/substitute.hpp

@@ -0,0 +1,27 @@
+//==================================================================================================
+/*
+  EVE - Expressive Vector Engine
+  Copyright : EVE Project Contributors
+  SPDX-License-Identifier: BSL-1.0
+*/
+//==================================================================================================
+#pragma once
+
+#include <concepts>
+#include <eve/module/core/constant/as_value.hpp>
+
+namespace eve
+{
+  //================================================================================================
+  //! @ingroup simd_concepts
+  //! @{
+  //! @concept substitute_for
+  //! @brief Specify that a type can be used as a substitute for another type after calling
+  //!        `eve::as_value` on a value of the first type.
+  //================================================================================================
+  template<typename T, typename U>
+  concept substitute_for = requires { as_value(T{}, as<U>{}); };
+  //================================================================================================
+  //! @}
+  //================================================================================================
+}

include/eve/module/core/regular/scan.hpp

@@ -10,13 +10,14 @@
 #include <eve/arch.hpp>
 #include <eve/detail/overload.hpp>
 #include <eve/concept/invocable.hpp>
+#include <eve/concept/substitute.hpp>
 
 namespace eve
 {
   template<typename Options>
   struct scan_t : callable<scan_t, Options>
   {
-    template<simd_value Wide, eve::monoid<Wide> Op, typename Zero>
+    template<simd_value Wide, eve::monoid<Wide> Op, eve::substitute_for<Wide> Zero>
     constexpr EVE_FORCEINLINE Wide operator()(Wide w, Op op, Zero z) const noexcept
       requires (requires { as_value(z, as<Wide>{}); })
     {