Try to fix clang compiler error.

Author: filipsajdak

include/cpp2util.h

@@ -2012,7 +2012,8 @@ constexpr auto is( X const& x ) -> auto
 }
 
 
-inline constexpr auto is( specialization_of_template<std::variant> auto const& x, auto&& value ) -> bool
+template <specialization_of_template<std::variant> X>
+inline constexpr auto is( X const& x, auto&& value ) -> bool
 {
     return type_find_if(x, [&]<typename It>(It const&) -> bool {
         if (x.index() == It::index) {
@@ -2026,16 +2027,16 @@ inline constexpr auto is( specialization_of_template<std::variant> auto const& x
     }) != std::variant_npos;
 }
 
-template< typename C >
-auto as(specialization_of_template<std::variant> auto&& x CPP2_SOURCE_LOCATION_PARAM_WITH_DEFAULT_AS) -> decltype(auto)
+template< typename C, specialization_of_template<std::variant> X >
+auto as( X&& x CPP2_SOURCE_LOCATION_PARAM_WITH_DEFAULT_AS) -> decltype(auto)
 {
-    constness_like_t<C, decltype(x)>* ptr = nullptr;
+    constness_like_t<C, X>* ptr = nullptr;
     type_find_if(CPP2_FORWARD(x), [&]<typename It>(It const&) -> bool {
         if constexpr (std::is_same_v< typename It::type, C >) { if (CPP2_FORWARD(x).index() ==  It::index) { ptr = &std::get<It::index>(x); return true; } }; 
         return false;
     });
     if (!ptr) { Throw( std::bad_variant_access(), "'as' cast failed for 'variant'"); }
-    return cpp2::forward_like<decltype(x)>(*ptr);
+    return cpp2::forward_like<X>(*ptr);
 }
 
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