Fix constexprness

Author: mborland

include/boost/decimal/detail/power_tables.hpp

@@ -9,31 +9,24 @@
 #include <boost/decimal/detail/type_traits.hpp>
 
 #ifndef BOOST_DECIMAL_BUILD_MODULE
-#include <array>
 #include <cstdint>
 #endif
 
 namespace boost {
 namespace decimal {
 namespace detail {
 
-BOOST_DECIMAL_CONSTEXPR_VARIABLE std::array<std::uint64_t, 20> powers_of_10 =
-{{
+BOOST_DECIMAL_CONSTEXPR_VARIABLE std::uint64_t powers_of_10[20] =
+{
          UINT64_C(1), UINT64_C(10), UINT64_C(100), UINT64_C(1000), UINT64_C(10000), UINT64_C(100000), UINT64_C(1000000),
          UINT64_C(10000000), UINT64_C(100000000), UINT64_C(1000000000), UINT64_C(10000000000), UINT64_C(100000000000),
          UINT64_C(1000000000000), UINT64_C(10000000000000), UINT64_C(100000000000000), UINT64_C(1000000000000000),
          UINT64_C(10000000000000000), UINT64_C(100000000000000000), UINT64_C(1000000000000000000), UINT64_C(10000000000000000000)
- }};
+ };
 
 template <typename T>
 constexpr auto pow10(T n) noexcept -> T
 {
-    BOOST_DECIMAL_IF_CONSTEXPR (detail::is_signed_v<T>)
-    {
-        BOOST_DECIMAL_ASSERT(n >= 0);
-    }
-    BOOST_DECIMAL_ASSERT(n <= 19);
-
     return static_cast<T>(powers_of_10[static_cast<std::size_t>(n)]);
 }
 
@@ -54,6 +47,27 @@ constexpr auto pow10(detail::uint128 n) noexcept -> detail::uint128
     return res;
 }
 
+#ifdef BOOST_DECIMAL_HAS_INT128
+
+template <>
+constexpr auto pow10(detail::uint128_t n) noexcept -> detail::uint128_t
+{
+    detail::uint128_t res {1};
+    if (n <= 19)
+    {
+        res = powers_of_10[static_cast<std::size_t>(n)];
+    }
+    else
+    {
+        res = powers_of_10[static_cast<std::size_t>(19)];
+        res *= powers_of_10[static_cast<std::size_t>(n - 19)];
+    }
+
+    return res;
+}
+
+#endif
+
 } // namespace detail
 } // namespace decimal
 } // namespace boost