[Bug libstdc++/118841] please do not use long double in generate_canonical when the range is small enough

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118841

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Maybe like this:

--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -3351,12 +3351,17 @@ namespace __detail
       static_assert(std::is_floating_point<_RealType>::value,
                    "template argument must be a floating point type");

+      using _MaxRealType
+       = __conditional_t<(std::numeric_limits<_RealType>::digits
+                            <= std::numeric_limits<double>::digits),
+                         double, long double>;
       const size_t __b
        = std::min(static_cast<size_t>(std::numeric_limits<_RealType>::digits),
                    __bits);
-      const long double __r = static_cast<long double>(__urng.max())
-                           - static_cast<long double>(__urng.min()) + 1.0L;
-      const size_t __log2r = std::log(__r) / std::log(2.0L);
+      const _MaxRealType __r = static_cast<_MaxRealType>(__urng.max())
+                                - static_cast<_MaxRealType>(__urng.min())
+                                + 1.0L;
+      const size_t __log2r = std::log(__r) / std::log((_MaxRealType)2.0);
       const size_t __m = std::max<size_t>(1UL,
                                          (__b + __log2r - 1UL) / __log2r);
       _RealType __ret;