Multithreading tests: Add dummy definition to keep compiler happy (#737)

* Multithreading tests: Add dummy definition to keep compiler happy

This type is instantiated by the deleted constructor whose original
purpose is to allow an implicit conversion from non-const iterator to
const iterator. When applied to the const iterator itself, it
enables an implicit conversion from garbage to const iterator.
The conversion is deleted, but the compiler still instantiates it,
so we define the type to keep everybody happy.

* Clarify the purpose of the specialization

Author: oldnewthing

test/test/multi_threaded_common.h

@@ -237,6 +237,17 @@ namespace concurrent_collections
     {
         return it += offset;
     }
+
+    // We have a constructor whose purpose is to offer an implicit conversion from
+    // non-const iterator to const iterator. When applied to a const
+    // iterator, it offers implicit construction from this garbage type
+    // to const iterator (triggering its instantiation), but then the constructor
+    // is deleted by SFINAE, so the type is never actually created at runtime.
+    // This specialization is necessary so that the garbage instantiation succeeds.
+    template<typename Container>
+    struct concurrency_checked_random_access_iterator<Container, void, void>
+    {
+    };
 #pragma endregion
 
     struct concurrency_guard