Firestore: Fix Windows build due to decltype of captured lambda (#10237)

Author: dconeybe

Firestore/core/src/local/local_store.cc

@@ -662,23 +662,18 @@ absl::optional<bundle::NamedQuery> LocalStore::GetNamedQuery(
 
 void LocalStore::ConfigureFieldIndexes(
     std::vector<FieldIndex> new_field_indexes) {
-  auto cmp = [](const FieldIndex& left, const FieldIndex& right) {
-    return FieldIndex::SemanticCompare(left, right) ==
-           util::ComparisonResult::Ascending;
-  };
-
   // This lambda function takes a rvalue vector as parameter,
   // then coverts it to a sorted set based on the compare function above.
-  auto convertToSet = [&](std::vector<FieldIndex>&& vec) {
-    std::set<FieldIndex, decltype(cmp)> result(cmp);
+  auto convertToSet = [](std::vector<FieldIndex>&& vec) {
+    std::set<FieldIndex, FieldIndex::SemanticLess> result;
     for (auto& index : vec) {
       result.insert(std::move(index));
     }
     return result;
   };
 
   return persistence_->Run("Configure indexes", [&] {
-    return util::DiffSets<FieldIndex, decltype(cmp)>(
+    return util::DiffSets<FieldIndex, FieldIndex::SemanticLess>(
         convertToSet(index_manager_->GetFieldIndexes()),
         convertToSet(std::move(new_field_indexes)), FieldIndex::SemanticCompare,
         [this](const model::FieldIndex& index) {

Firestore/core/src/model/field_index.h

@@ -285,6 +285,21 @@ class FieldIndex {
   /** Returns the ArrayContains/ArrayContainsAny segment for this index. */
   absl::optional<Segment> GetArraySegment() const;
 
+  /**
+   * A type that can be used as the "Compare" template parameter of ordered
+   * collections to have the elements ordered using
+   * `FieldIndex::SemanticCompare()`.
+   *
+   * Example:
+   * std::set<FieldIndex, FieldIndex::SemanticLess> result;
+   */
+  struct SemanticLess {
+    bool operator()(const FieldIndex& left, const FieldIndex& right) const {
+      return FieldIndex::SemanticCompare(left, right) ==
+             util::ComparisonResult::Ascending;
+    }
+  };
+
  private:
   friend bool operator==(const FieldIndex& lhs, const FieldIndex& rhs);
   friend bool operator!=(const FieldIndex& lhs, const FieldIndex& rhs);

Firestore/core/test/unit/local/leveldb_local_store_test.cc

@@ -69,12 +69,7 @@ std::unique_ptr<LocalStoreTestHelper> Factory() {
 // This lambda function takes a rvalue vector as parameter,
 // then coverts it to a sorted set based on the compare function.
 auto convertToSet = [](std::vector<FieldIndex>&& vec) {
-  auto cmp = [](const FieldIndex& left, const FieldIndex& right) {
-    return FieldIndex::SemanticCompare(left, right) ==
-           util::ComparisonResult::Ascending;
-  };
-
-  std::set<FieldIndex, decltype(cmp)> result(cmp);
+  std::set<FieldIndex, FieldIndex::SemanticLess> result;
   for (auto& index : vec) {
     result.insert(std::move(index));
   }