[CUDA] Add tests to ensure that std::min/max can be called from __host__ __device__ functions.

[jlebar]

Summary

Tests for D48036 / PR37753.

External/CUDA/algorithm.cu

 #include <assert.h>
 #include <algorithm>
 #include <functional>
 #include <stdio.h>
 
 __device__ void min() {
   assert(std::min(0, 1) == 0);
 }
+__host__ __device__ void min_hd() {
+  assert(std::min(0, 1) == 0);
+}
 
 __device__ void max() {
   assert(std::max(0, 1) == 1);
 }
+__host__ __device__ void max_hd() {
+  assert(std::max(0, 1) == 1);
+}
 
 // Clang has device-side shims implementing std::min and std::max for scalars
 // starting in C++11, but doesn't implement minimax or std::min/max on
 // initializer_lists until C++14, when it gets these for free from the standard
 // library (because they're constexpr).
 __device__ void cpp14_tests() {
 #if __cplusplus >= 201402L && STDLIB_VERSION >= 2014
   assert(std::greater<int>()(1, 0));
   assert(std::min({5, 1, 10}) == 1);
   assert(std::max({5, 1, 10}, std::less<int>()) == 10);
 
   assert(std::minmax(1, 0).first == 0);
   assert(std::minmax(1, 0).second == 1);
   assert(std::minmax({0, 10, -10, 100}, std::less<int>()).first == -10);
   assert(std::minmax({0, 10, -10, 100}, std::less<int>()).second == 100);
 #endif
 }
 
+// Same tests as cpp14_tests, but from a host-device context.
+__host__ __device__ void cpp14_tests_hd() {
+#if __cplusplus >= 201402L && STDLIB_VERSION >= 2014
+  assert(std::greater<int>()(1, 0));
+  assert(std::min({5, 1, 10}) == 1);
+  assert(std::max({5, 1, 10}, std::less<int>()) == 10);
+
+  assert(std::minmax(1, 0).first == 0);
+  assert(std::minmax(1, 0).second == 1);
+  assert(std::minmax({0, 10, -10, 100}, std::less<int>()).first == -10);
+  assert(std::minmax({0, 10, -10, 100}, std::less<int>()).second == 100);
+#endif
+}
+
 __global__ void kernel() {
   min();
+  min_hd();
   max();
+  max_hd();
   cpp14_tests();
+  cpp14_tests_hd();
 }
 
 int main() {
   kernel<<<32, 32>>>();
   cudaError_t err = cudaDeviceSynchronize();
   if (err != cudaSuccess) {
     printf("CUDA error %d\n", (int)err);
     return 1;
   }
+
+  min_hd();
+  max_hd();
+  cpp14_tests_hd();
+
   printf("Success!\n");
   return 0;
 }
 
 #else
 
 #include <stdio.h>
 int main() {
   printf("Success!\n");
   return 0;
 }
 
 #endif