Reapply "SERVER-30991 Introduce MatchExpression::optimize()."

This patch undoes the revert in
3cf4e0593c394dd7eb45d8000d76b5dc73a3f425 and includes minor changes to
fix the compiler problem that resulted in the previous revert.

1 files changed, 14 insertions, 0 deletions

diff --git a/src/mongo/db/matcher/expression_arity.h b/src/mongo/db/matcher/expression_arity.h
index dc43269a91a..e6f71395941 100644
--- a/src/mongo/db/matcher/expression_arity.h
+++ b/src/mongo/db/matcher/expression_arity.h
@@ -147,6 +147,20 @@ protected:
     }
 
 private:
+    ExpressionOptimizerFunc getOptimizer() const final {
+        return [](std::unique_ptr<MatchExpression> expression) {
+            for (auto& subExpression :
+                 static_cast<FixedArityMatchExpression&>(*expression)._expressions) {
+                // Since 'subExpression' is a reference to a member of the
+                // FixedArityMatchExpression's child array, this assignment replaces the original
+                // child with the optimized child.
+                subExpression = MatchExpression::optimize(std::move(subExpression));
+            }
+
+            return expression;
+        };
+    }
+
     std::array<std::unique_ptr<MatchExpression>, nargs> _expressions;
 };