Improve __float128 support

include/fmt/format-inl.h

@@ -217,8 +217,10 @@ template <typename F> struct basic_fp {
   template <typename Float> FMT_CONSTEXPR basic_fp(Float n) { assign(n); }
 
   template <typename Float>
-  using is_supported = bool_constant<std::numeric_limits<Float>::is_iec559 &&
-                                     std::numeric_limits<Float>::digits <= 113>;
+  using is_supported =
+      bool_constant<(std::numeric_limits<Float>::is_iec559 &&
+                     std::numeric_limits<Float>::digits <= 113) ||
+                    is_float128<Float>::value>;
 
   // Assigns d to this and return true iff predecessor is closer than successor.
   template <typename Float, FMT_ENABLE_IF(is_supported<Float>::value)>
@@ -229,7 +231,7 @@ template <typename F> struct basic_fp {
                                       << detail::num_significand_bits<Float>();
     const carrier_uint significand_mask = implicit_bit - 1;
     auto u = bit_cast<carrier_uint>(n);
-    f = static_cast<uint64_t>(u & significand_mask);
+    f = static_cast<F>(u & significand_mask);
     int biased_e = static_cast<int>((u & exponent_mask<Float>()) >>
                                     detail::num_significand_bits<Float>());
     // The predecessor is closer if n is a normalized power of 2 (f == 0) other

include/fmt/format.h

@@ -1286,14 +1286,19 @@ template <typename T> struct decimal_fp {
 template <typename T> FMT_API auto to_decimal(T x) noexcept -> decimal_fp<T>;
 }  // namespace dragonbox
 
+// Returns true iff Float has the implicit bit which is not stored.
 template <typename Float> constexpr bool has_implicit_bit() {
+  // An 80-bit FP number has a 64-bit significand an no implicit bit.
   return std::numeric_limits<Float>::digits != 64;
 }
 
-// Returns the number of significand bits in Float excluding the implicit bit.
+// Returns the number of significand bits stored in Float. The implicit bit is
+// not counted since it is not stored.
 template <typename Float> constexpr int num_significand_bits() {
-  return std::numeric_limits<Float>::digits -
-         (has_implicit_bit<Float>() ? 1 : 0);
+  // std::numeric_limits may not support __float128.
+  return is_float128<Float>() ? 112
+                              : (std::numeric_limits<Float>::digits -
+                                 (has_implicit_bit<Float>() ? 1 : 0));
 }
 
 template <typename Float>