Use Layout::WithStaticSizes in btree.

PiperOrigin-RevId: 617877687
Change-Id: I29c52f9288f099255c4adb7c1f9fa8831ac55b05

absl/container/internal/btree.h

@@ -580,14 +580,12 @@ class btree_node {
   using layout_type =
       absl::container_internal::Layout<btree_node *, uint32_t, field_type,
                                        slot_type, btree_node *>;
-  constexpr static size_type SizeWithNSlots(size_type n) {
-    return layout_type(
+  using leaf_layout_type = typename layout_type::template WithStaticSizes<
       /*parent*/ 1,
       /*generation*/ BtreeGenerationsEnabled() ? 1 : 0,
-               /*position, start, finish, max_count*/ 4,
-               /*slots*/ n,
-               /*children*/ 0)
-        .AllocSize();
+      /*position, start, finish, max_count*/ 4>;
+  constexpr static size_type SizeWithNSlots(size_type n) {
+    return leaf_layout_type(/*slots*/ n, /*children*/ 0).AllocSize();
   }
   // A lower bound for the overhead of fields other than slots in a leaf node.
   constexpr static size_type MinimumOverhead() {
@@ -619,27 +617,22 @@ class btree_node {
   constexpr static size_type kNodeSlots =
       kNodeTargetSlots >= kMinNodeSlots ? kNodeTargetSlots : kMinNodeSlots;
 
+  using internal_layout_type = typename layout_type::template WithStaticSizes<
+      /*parent*/ 1,
+      /*generation*/ BtreeGenerationsEnabled() ? 1 : 0,
+      /*position, start, finish, max_count*/ 4, /*slots*/ kNodeSlots,
+      /*children*/ kNodeSlots + 1>;
+
   // The node is internal (i.e. is not a leaf node) if and only if `max_count`
   // has this value.
   constexpr static field_type kInternalNodeMaxCount = 0;
 
-  constexpr static layout_type Layout(const size_type slot_count,
-                                      const size_type child_count) {
-    return layout_type(
-        /*parent*/ 1,
-        /*generation*/ BtreeGenerationsEnabled() ? 1 : 0,
-        /*position, start, finish, max_count*/ 4,
-        /*slots*/ slot_count,
-        /*children*/ child_count);
-  }
   // Leaves can have less than kNodeSlots values.
-  constexpr static layout_type LeafLayout(
+  constexpr static leaf_layout_type LeafLayout(
       const size_type slot_count = kNodeSlots) {
-    return Layout(slot_count, 0);
-  }
-  constexpr static layout_type InternalLayout() {
-    return Layout(kNodeSlots, kNodeSlots + 1);
+    return leaf_layout_type(slot_count, 0);
   }
+  constexpr static auto InternalLayout() { return internal_layout_type(); }
   constexpr static size_type LeafSize(const size_type slot_count = kNodeSlots) {
     return LeafLayout(slot_count).AllocSize();
   }