work around miscompile in parallel-map test

Summary: Some versions of gcc miscompile the code in the parallel-map exception test. Work around the miscompile by adding an extra log line.

Reviewed By: Orvid

Differential Revision: D26509079

fbshipit-source-id: 1474437d96aeabad0294bcb395b7baec229eb9ba

folly/gen/test/ParallelMapTest.cpp

@@ -156,6 +156,9 @@ TEST(Pmap, Rvalues) {
 }
 
 TEST(Pmap, Exception) {
+#if __GNUC__ == 7 && __GNUC_MINOR__ == 5 && !__clang__
+  LOG(INFO) << "some versions of gcc miscompile the code below without this";
+#endif
   std::vector<char const*> input{"a"};
   EXPECT_THROW(from(input) | pmap(To<int>()) | count, std::runtime_error);
 }