Disallow using a hash function whose return type is smaller than size_t.

Swisstable relies on the hash having good entropy in both the high and low bits.

PiperOrigin-RevId: 786854836
Change-Id: I6fc7041534984a0e937a11f50a2b478390a1a509

absl/container/internal/raw_hash_set.h

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 5fe3367..679d68c 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -1915,6 +1915,13 @@
   auto KeyTypeCanBeHashed(const Hash& h, const key_type& k) -> decltype(h(k));
   auto KeyTypeCanBeEq(const Eq& eq, const key_type& k) -> decltype(eq(k, k));
 
+  // Try to be helpful when the hasher returns an unreasonable type.
+  using key_hash_result =
+      absl::remove_cvref_t<decltype(std::declval<const Hash&>()(
+          std::declval<const key_type&>()))>;
+  static_assert(sizeof(key_hash_result) >= sizeof(size_t),
+                "`Hash::operator()` should return a `size_t`");
+
   using AllocTraits = absl::allocator_traits<allocator_type>;
   using SlotAlloc = typename absl::allocator_traits<
       allocator_type>::template rebind_alloc<slot_type>;