Listing 9.3 - Compile Error

[ITHelpDec]

A return is needed when calling the two-parameter version of accumulate_block() inside of the lambda function.

futures[i] = pool.submit([=] () {
-    accumulate_block<_ForwardIt, _Tp>()(block_start, block_end);
+    return accumulate_block<_ForwardIt, _Tp>()(block_start, block_end);
});

Without it, we receive the following error:

No viable overloaded '='

The listing in this repo (here) also doesn't mirror what was written in the book, so I've first brought them in sync before applying the correction.