view: prefer static_assert over SFINAE for extract

This produces more readable error messages, and we are not using
`extract` in a context where we need to disambiguate calls anyway.

view.hpp

@@ -724,20 +724,24 @@ namespace cruft {
     /// It is assumed the user has taken care of alignment concerns
     template <
         typename ValueT,
-        typename WordT,
+        typename WordT
-        typename = std::enable_if_t<
-            // Only allow calls if the value is a multiple of the word size.
-            // It's useful to allow non-unit words for areas like TCP/IP which
-            // tend to include protocols that utilise u16 words.
-            sizeof (ValueT) % sizeof (WordT) == 0 &&
-            // If WordT is const then ValueT must be const. Otherwise the user
-            // is free to choose.
-            std::is_const_v<WordT> ? std::is_const_v<ValueT> : true
-        >
     >
     ValueT&
     extract (view<WordT*> &buffer)
     {
+        // Only allow calls if the value is a multiple of the word size.
+        // It's useful to allow non-unit words for areas like TCP/IP which
+        // tend to include protocols that utilise u16 words.
+        static_assert (
+            sizeof (ValueT) % sizeof (WordT) == 0,
+            "The value type must be a multiple of the word size"
+        );
+
+        static_assert (
+            !std::is_const_v<WordT> or std::is_const_v<ValueT>,
+            "buffer and output types must have matching constness"
+        );
+
         if (unlikely (sizeof (ValueT) > buffer.size () * sizeof (WordT)))
             throw std::runtime_error ("insufficient data for extraction");