implement feedback from brunocodutra#100 (comment)

Author: ecrypa

include/metal/detail/sfinae.hpp

@@ -58,15 +58,6 @@ namespace metal {
         template<template<typename...> class expr, typename... vals>
         using call = expr<vals...>;
 #endif
-
-        template<class... vals>
-        struct _dcaller {
-            template<template<class...> class expr>
-            using type = expr<vals...>;
-        };
-
-        template<template<class...> class expr, class... vals>
-        using dcall = typename _dcaller<vals...>::template type<expr>;
     }
     /// \endcond
 }

include/metal/list/min_element.hpp

@@ -2,31 +2,32 @@
 #define METAL_LIST_MIN_ELEMENT_HPP
 
 #include "../config.hpp"
-#include "../detail/sfinae.hpp"
 #include "../lambda/apply.hpp"
+#include "../lambda/invoke.hpp"
 #include "../lambda/lambda.hpp"
 #include "../number/if.hpp"
+#include "../number/number.hpp"
 #include "../value/fold_left.hpp"
 
 namespace metal {
     namespace detail {
-
         template<class lbd>
-        struct _min_element {};
-
-        template<template<class...> class expr>
-        struct _min_element<lambda<expr>> {
+        struct _min_element {
             template<class x, class y>
-            using combiner = if_<dcall<expr, y, x>, y, x>;
+            using combiner = if_<invoke<lbd, y, x>, y, x>;
+            // Combiner must use `invoke` in order to support SFINAE. It is
+            // tempting to extract the `expr` from `lbd` and use `expr<y, x>`,
+            // but that is not SFINAE-friendly.
 
             template<class... vals>
             using type = fold_left<lambda<combiner>, vals...>;
         };
     }
 
     template<class seq, class lbd>
-    using min_element =
-        apply<lambda<detail::_min_element<lbd>::template type>, seq>;
+    using min_element = apply<
+        lambda<detail::_min_element<if_<is_lambda<lbd>, lbd>>::template type>,
+        seq>;
 }
 
 #endif