Switching to what is hopefully a better fix for the following CUDA error

error: calling a constexpr host function("log1p") from a device function("cuda_log1pexp") is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.

The error only happens with some versions of CUDA.

dlib/CMakeLists.txt

@@ -633,14 +633,6 @@ if (NOT TARGET dlib)
                endif()
             endif()
 
-            if (CUDA_VERSION VERSION_LESS 10.3)
-               # To avoid this error from nvcc:  
-               #    error: calling a constexpr __host__ function("log1p") from a __device__ function("cuda_log1pexp") 
-               #    is not allowed. The experimental flag '--expt-relaxed-constexpr' can be used to allow this.
-               # which is a bug in nvcc in this version of cuda.
-               list(APPEND FLAGS_FOR_NVCC "--expt-relaxed-constexpr")
-            endif()
-
             set(CUDA_HOST_COMPILATION_CPP ON)
             # Note that we add __STRICT_ANSI__ to avoid freaking out nvcc with gcc specific
             # magic in the standard C++ header files (since nvcc uses gcc headers on

dlib/cuda/cuda_dlib.cu

@@ -1759,9 +1759,9 @@ namespace dlib
             if (x <= -18)
                 return std::exp(x);
             else if (-18 < x && x <= 9)
-                return std::log1p(std::exp(x));
+                return std::log1pf(std::exp(x));
             else if (9 < x && x <= 16)
-                return x + std::exp(-x);
+                return x + expf(-x);
             else
                 return x;
         }