Add ilogb support to decimal128

Author: mborland

include/boost/decimal/decimal128.hpp

@@ -214,6 +214,12 @@ class decimal128 final
 
     friend constexpr auto d128_mod_impl(decimal128 lhs, decimal128 rhs, const decimal128& q, decimal128& r) noexcept -> void;
 
+    template <typename T>
+    friend constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>;
+
+    template <typename T>
+    friend constexpr auto logb(T num) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, T>;
+
 public:
     // 3.2.4.1 construct/copy/destroy
     constexpr decimal128() noexcept = default;

include/boost/decimal/detail/cmath/ilogb.hpp

@@ -11,9 +11,8 @@
 #include <boost/decimal/fwd.hpp> // NOLINT(llvm-include-order)
 #include <boost/decimal/detail/type_traits.hpp>
 
-namespace boost { namespace decimal {
-
-// TODO(mborland): Allow conversion between decimal types via a promotion system
+namespace boost {
+namespace decimal {
 
 template <typename T>
 constexpr auto ilogb(T d) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<T>, int>