Fix compilation on platforms with exotic double (#878)

include/fmt/format-inl.h

@@ -513,7 +513,8 @@ FMT_FUNC void grisu2_gen_digits(
   }
 }
 
-FMT_FUNC void grisu2_format_positive(double value, char *buffer, size_t &size,
+template <typename Double>
+FMT_FUNC void grisu2_format_positive(Double value, char *buffer, size_t &size,
                                      int &dec_exp) {
   FMT_ASSERT(value > 0, "value is nonpositive");
   fp fp_value(value);
@@ -640,8 +641,10 @@ FMT_FUNC void grisu2_prettify(char *buffer, size_t &size, int exp,
 
 // Formats a nonnegative value using Grisu2 algorithm. Grisu2 doesn't give any
 // guarantees on the shortness of the result.
-FMT_FUNC void grisu2_format(double value, char *buffer, size_t &size, char type,
-                            int precision, bool write_decimal_point) {
+template <typename Double>
+FMT_FUNC typename std::enable_if<sizeof(Double) == sizeof(uint64_t)>::type
+  grisu2_format(Double value, char *buffer, size_t &size, char type,
+                int precision, bool write_decimal_point) {
   FMT_ASSERT(value >= 0, "value is negative");
   int dec_exp = 0;  // K in Grisu.
   if (value > 0) {

include/fmt/format.h

@@ -293,8 +293,13 @@ inline bool use_grisu() {
 
 // Formats value using Grisu2 algorithm:
 // https://www.cs.tufts.edu/~nr/cs257/archive/florian-loitsch/printf.pdf
-FMT_API void grisu2_format(double value, char *buffer, size_t &size, char type,
-                           int precision, bool write_decimal_point);
+template <typename Double>
+FMT_API typename std::enable_if<sizeof(Double) == sizeof(uint64_t)>::type
+  grisu2_format(Double value, char *buffer, size_t &size, char type,
+                int precision, bool write_decimal_point);
+template <typename Double>
+inline typename std::enable_if<sizeof(Double) != sizeof(uint64_t)>::type
+  grisu2_format(Double, char *, size_t &, char, int, bool) {}
 
 template <typename Allocator>
 typename Allocator::value_type *allocate(Allocator& alloc, std::size_t n) {

test/format-impl-test.cc

@@ -102,6 +102,11 @@ TEST(FPTest, GetCachedPower) {
   }
 }
 
+TEST(FPTest, Grisu2FormatCompilesWithNonIEEEDouble) {
+  size_t size = 0;
+  fmt::internal::grisu2_format(4.2f, FMT_NULL, size, 0, 0, false);
+}
+
 template <typename T>
 struct ValueExtractor: fmt::internal::function<T> {
   T operator()(T value) {