westonpace commented on a change in pull request #9892:
URL: https://github.com/apache/arrow/pull/9892#discussion_r608052063



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File path: cpp/src/arrow/dataset/scanner.h
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@@ -32,6 +32,7 @@
 #include "arrow/memory_pool.h"
 #include "arrow/type_fwd.h"
 #include "arrow/util/async_generator.h"
+#include "arrow/util/thread_pool.h"

Review comment:
       I tried and failed removing `async_generator.h` before.  The crux of the 
issue was...
   
   `AsyncGenerator<T>` is a typedef of `std::function<Future<T>()>` and not a 
proper class in its own right (in fact, `async_generator.h` hides all generator 
classes behind `MakeXYZGenerator` and so it doesn't actually need to export any 
types beyond `std::function`).
   
   `std::function<Future<T>()>` requires `<functional>`.
   
   We don't want `<functional>` in a `type_fwd`.
   
   I'm open to suggestions.




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