Upgrade c++14 to c++17 in cmake (#15073)

Author: cherylEnkidu

Firestore/core/src/local/leveldb_remote_document_cache.cc

@@ -193,8 +193,8 @@ MutableDocumentMap LevelDbRemoteDocumentCache::GetAllExisting(
   tasks.AwaitAll();
 
   MutableDocumentMap map;
-  for (const auto& entry : results.Result()) {
-    map = map.insert(entry.first, entry.second);
+  for (const auto& [key, doc] : results.Result()) {
+    map = map.insert(key, doc);
   }
   return map;
 }

Firestore/core/test/unit/util/iterator_adaptors_test.cc

@@ -82,8 +82,14 @@ class IteratorAdaptorTest : public testing::Test {
   }
 
   template <typename T>
-  class InlineStorageIter : public std::iterator<std::input_iterator_tag, T> {
+  class InlineStorageIter {
    public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = T;
+    using difference_type = std::ptrdiff_t;
+    using pointer = T*;
+    using reference = T&;
+
     T* operator->() const {
       return get();
     }
@@ -567,9 +573,14 @@ TEST_F(IteratorAdaptorTest, IteratorPtrHasRandomAccessMethods) {
   EXPECT_EQ(88, value2);
 }
 
-class MyInputIterator
-    : public std::iterator<std::input_iterator_tag, const int*> {
+class MyInputIterator {
  public:
+  using iterator_category = std::input_iterator_tag;
+  using value_type = const int*;
+  using difference_type = std::ptrdiff_t;
+  using pointer = const int**;
+  using reference = const int*;
+
   explicit MyInputIterator(int* x) : x_(x) {
   }
   const int* operator*() const {

cmake/compiler_setup.cmake

@@ -16,8 +16,8 @@ include(CheckCXXCompilerFlag)
 
 # C++ Compiler setup
 
-# We use C++14
-set(CMAKE_CXX_STANDARD 14)
+# We use C++17
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)