fix: replace uses of deprecated has_formatter

use is_formattable instead.

has_formatter was never a good API, since it only asked if the type has
a `formatter` specialization, but a type can also be formattable if
it provides an implicit conversion to a type which does.

The newer API takes care of the that problem.

as a drive-by, provides format_as free function for dom::Value instead of
a fmt::formatter specialization.

Author: mizvekov

include/mrdocs/Dom/Value.hpp

@@ -527,6 +527,8 @@ class MRDOCS_DECL
     toString(Value const& value);
 };
 
+inline auto format_as(Value value) { return toString(value); }
+
 //------------------------------------------------
 
 namespace JSON
@@ -847,19 +849,4 @@ safeString(SV const& str) {
 #include <mrdocs/Dom/Function.ipp>
 #include <mrdocs/Dom/Object.ipp>
 
-//------------------------------------------------
-
-template<>
-struct fmt::formatter<clang::mrdocs::dom::Value>
-    : public fmt::formatter<std::string>
-{
-    auto format(
-        clang::mrdocs::dom::Value const& value,
-        fmt::format_context& ctx) const
-    {
-        return fmt::formatter<std::string>::format(
-            toString(value), ctx);
-    }
-};
-
 #endif

include/mrdocs/Support/Handlebars.hpp

@@ -218,7 +218,7 @@ class MRDOCS_DECL OutputRef
         @return A reference to this object
      */
     template <class T>
-    requires fmt::has_formatter<T, fmt::format_context>::value
+    requires fmt::formattable<T>
     friend
     OutputRef&
     operator<<( OutputRef& os, T v )

src/test_suite/detail/decomposer.hpp

@@ -53,7 +53,7 @@ namespace test_suite::detail
             out += '\"';
         }
 #ifdef MRDOCS_TEST_HAS_FMT
-        if constexpr (fmt::has_formatter<T, fmt::format_context>::value) {
+        if constexpr (fmt::is_formattable<T>::value) {
             out += fmt::format("{}", value);
         } else
 #endif