final classes can't be empty-base-optimized

For that we need [[no_unique_address]].

Actually wait, I think maybe we have [[no_unique_address]]! Maybe we can
get rid of compressed_pair entirely...

ctl/compressed_pair.h

@@ -3,6 +3,7 @@
 #ifndef COSMOPOLITAN_CTL_COMPRESSED_PAIR_H_
 #define COSMOPOLITAN_CTL_COMPRESSED_PAIR_H_
 #include <__type_traits/is_empty.h>
+#include <__type_traits/is_final.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
 #include <__utility/swap.h>
@@ -12,7 +13,7 @@ namespace ctl {
 namespace __ {
 
 template<typename T>
-concept Empty = std::is_empty_v<T>;
+concept EmptyBase = std::is_empty_v<T> && !std::is_final_v<T>;
 
 template<typename T, int I>
 struct pair_elem
@@ -37,7 +38,7 @@ struct pair_elem
     T t;
 };
 
-template<Empty T, int I>
+template<EmptyBase T, int I>
 struct pair_elem<T, I> : private T
 {
     using value_type = T;

test/ctl/unique_ptr_test.cc

@@ -54,6 +54,21 @@ struct SetsGDeleter
     }
 };
 
+struct FinalDeleter final
+{
+    void operator()(auto*) const noexcept
+    {
+    }
+};
+
+struct StatefulDeleter
+{
+    char state;
+    void operator()(auto*) const noexcept
+    {
+    }
+};
+
 static_assert(sizeof(Ptr<int, SetsGDeleter>) == sizeof(int*));
 
 struct SetsGCtor
@@ -185,6 +200,13 @@ main()
     }
 #endif
 
+    {
+        int a;
+        // Should compile.
+        Ptr<int, FinalDeleter> x(&a);
+        Ptr<int, StatefulDeleter> y(&a);
+    }
+
     // next is 18
 
     return 0;