flat_map: Avoid double-templating Pair

As is, the type is a bit obnoxious to name, because you have to use
either FlatMap<K, V, 1>::Pair<K, V> or FlatMap<K, V, 1>::value_type.

Because FlatMap doesn't have any copy semantics, you are pretty much
required to use constructors that take the expected std::array, should a
class want to decorate this type.

Change-Id: I908fdf7d0f4502932d2c1244e8e8b5dc593267a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/102130
Commit-Queue: Anqi Dong <anqid@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>

pw_containers/public/pw_containers/flat_map.h

@@ -21,25 +21,25 @@
 
 namespace pw::containers {
 
+// Define and use a custom Pair object. This is because std::pair does not
+// support constexpr assignment until C++20. The assignment is needed since
+// the array of pairs will be sorted in the constructor (if not already).
+template <typename First, typename Second>
+struct Pair {
+  First first;
+  Second second;
+};
+
 // A simple, fixed-size associative array with lookup by key or value.
 //
-// FlatMaps are initialized with a std::array of FlatMap::Pair objects:
+// FlatMaps are initialized with a std::array of Pair<K, V> objects:
 //   FlatMap<int, int> map({{{1, 2}, {3, 4}}});
 //
 // The keys do not need to be sorted as the constructor will sort the items
 // if need be.
 template <typename Key, typename Value, size_t kArraySize>
 class FlatMap {
  public:
-  // Define and use a custom Pair object. This is because std::pair does not
-  // support constexpr assignment until C++20. The assignment is needed since
-  // the array of pairs will be sorted in the constructor (if not already).
-  template <typename First, typename Second>
-  struct Pair {
-    First first;
-    Second second;
-  };
-
   using key_type = Key;
   using mapped_type = Value;
   using value_type = Pair<key_type, mapped_type>;