ppltasks.cpp: Fix bincompat for VS 2015 (microsoft#3255)

stl/src/ppltasks.cpp

@@ -287,6 +287,13 @@ namespace Concurrency {
         }
 
         _CRTIMP2 bool __cdecl _Task_impl_base::_IsNonBlockingThread() {
+// TRANSITION, ABI: This preprocessor directive attempts to fix VSO-1684985 (a bincompat issue affecting VS 2015 code)
+// while preserving as much of GH-2654 as possible. When we can break ABI, we should:
+// * Remove this preprocessor directive - it should be unnecessary after <ppltasks.h> was changed on 2018-01-12.
+// * In <ppltasks.h>, reconsider whether _Task_impl_base::_Wait() should throw invalid_operation;
+//   it's questionable whether that's conforming, and if users want to block their UI threads, we should let them.
+// * Investigate whether we can avoid the ppltasks dependency entirely, making all of these issues irrelevant.
+#if defined(_CRT_APP) || defined(UNDOCKED_WINDOWS_UCRT)
             APTTYPE _AptType;
             APTTYPEQUALIFIER _AptTypeQualifier;
 
@@ -314,6 +321,8 @@ namespace Concurrency {
                     break;
                 }
             }
+#endif // defined(_CRT_APP) || defined(UNDOCKED_WINDOWS_UCRT)
+
             return false;
         }
     } // namespace details