diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index feb7815d..9baf2ca1 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -216,11 +216,10 @@ template <typename F> struct basic_fp {
   template <typename Float> FMT_CONSTEXPR basic_fp(Float n) { assign(n); }
 
   // Assigns n to this and return true iff predecessor is closer than successor.
-  template <typename Float> FMT_CONSTEXPR auto assign(Float n) -> bool {
-    static_assert((std::numeric_limits<Float>::is_iec559 &&
-                   std::numeric_limits<Float>::digits <= 113) ||
-                      is_float128<Float>::value,
-                  "unsupported FP");
+  template <typename Float,
+            FMT_ENABLE_IF(std::numeric_limits<Float>::is_iec559)>
+  FMT_CONSTEXPR auto assign(Float n) -> bool {
+    static_assert(std::numeric_limits<Float>::digits <= 113, "unsupported FP");
     // Assume Float is in the format [sign][exponent][significand].
     using carrier_uint = typename dragonbox::float_info<Float>::carrier_uint;
     const auto num_float_significand_bits =
@@ -231,8 +230,8 @@ template <typename F> struct basic_fp {
     f = static_cast<F>(u & significand_mask);
     auto biased_e = static_cast<int>((u & exponent_mask<Float>()) >>
                                      num_float_significand_bits);
-    // The predecessor is closer if n is a normalized power of 2 (f == 0) other
-    // than the smallest normalized number (biased_e > 1).
+    // The predecessor is closer if n is a normalized power of 2 (f == 0)
+    // other than the smallest normalized number (biased_e > 1).
     auto is_predecessor_closer = f == 0 && biased_e > 1;
     if (biased_e == 0)
       biased_e = 1;  // Subnormals use biased exponent 1 (min exponent).
@@ -242,6 +241,13 @@ template <typename F> struct basic_fp {
     if (!has_implicit_bit<Float>()) ++e;
     return is_predecessor_closer;
   }
+
+  template <typename Float,
+            FMT_ENABLE_IF(!std::numeric_limits<Float>::is_iec559)>
+  FMT_CONSTEXPR auto assign(Float n) -> bool {
+    static_assert(std::numeric_limits<double>::is_iec559, "unsupported FP");
+    return assign(static_cast<double>(n));
+  }
 };
 
 using fp = basic_fp<unsigned long long>;
diff --git a/include/fmt/format.h b/include/fmt/format.h
index fe74db64..d6a77cc2 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -990,11 +990,12 @@ constexpr auto is_negative(T) -> bool {
   return false;
 }
 
-template <typename T> constexpr auto is_supported_floating_point(T) -> bool {
-  return (std::is_same<T, float>() && FMT_USE_FLOAT) ||
-         (std::is_same<T, double>() && FMT_USE_DOUBLE) ||
-         (std::is_same<T, long double>() && FMT_USE_LONG_DOUBLE) ||
-         is_float128<T>();
+template <typename T>
+FMT_CONSTEXPR auto is_supported_floating_point(T) -> bool {
+  if (std::is_same<T, float>()) return FMT_USE_FLOAT;
+  if (std::is_same<T, double>()) return FMT_USE_DOUBLE;
+  if (std::is_same<T, long double>()) return FMT_USE_LONG_DOUBLE;
+  return true;
 }
 
 // Smallest of uint32_t, uint64_t, uint128_t that is large enough to
@@ -2345,14 +2346,14 @@ template <typename T, FMT_ENABLE_IF(std::is_floating_point<T>::value&&
 FMT_CONSTEXPR20 bool isfinite(T value) {
   constexpr T inf = T(std::numeric_limits<double>::infinity());
   if (is_constant_evaluated())
-    return !isnan(value) && value != inf && value != -inf;
+    return !detail::isnan(value) && value != inf && value != -inf;
   return std::isfinite(value);
 }
 template <typename T, FMT_ENABLE_IF(!has_isfinite<T>::value)>
 FMT_CONSTEXPR bool isfinite(T value) {
   T inf = T(std::numeric_limits<double>::infinity());
   // std::isfinite doesn't support __float128.
-  return !isnan(value) && value != inf && value != -inf;
+  return !detail::isnan(value) && value != inf && value != -inf;
 }
 
 template <typename T, FMT_ENABLE_IF(is_floating_point<T>::value)>
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index f6f2860d..bea9bd67 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -361,6 +361,37 @@ TEST(format_impl_test, write_float128) {
 }
 #endif
 
+struct double_double {
+  double a;
+  double b;
+
+  explicit constexpr double_double(double a_val = 0, double b_val = 0)
+      : a(a_val), b(b_val) {}
+
+  operator double() const { return a + b; }
+  auto operator-() const -> double_double { return double_double(-a, -b); }
+};
+
+bool operator>=(const double_double& lhs, const double_double& rhs) {
+  return lhs.a + lhs.b >= rhs.a + rhs.b;
+}
+
+namespace std {
+template <> struct is_floating_point<double_double> : std::true_type {};
+template <> struct numeric_limits<double_double> {
+  static constexpr bool is_iec559 = false;
+  static constexpr int digits = 106;
+};
+}  // namespace std
+
+TEST(format_impl_test, write_double_double) {
+  auto s = std::string();
+  fmt::detail::write<char>(std::back_inserter(s), double_double(42), {});
+#ifndef _MSC_VER  // MSVC has an issue with specializing is_floating_point.
+  EXPECT_EQ(s, "42");
+#endif
+}
+
 #ifdef _WIN32
 #  include <windows.h>