Update the minimum version from C++14 to C++17

Firestore/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- [changed] Drop C++14 support, Firestore SDK now requires at least C++17.
+
 # 11.12.0
 - [fixed] Fixed the `null` value handling in `isNotEqualTo` and `notIn` filters.
 

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)