From b7e1c1d59c374f1707d7a7207211a5f8a438ee1d Mon Sep 17 00:00:00 2001
From: Casey Bodley <cbodley@redhat.com>
Date: Tue, 14 Nov 2023 14:43:25 -0500
Subject: [PATCH] common: add generate_test_instances() for std::variant

Signed-off-by: Casey Bodley <cbodley@redhat.com>
---
 src/common/versioned_variant.h            | 19 +++++++++++++++++++
 src/test/common/test_versioned_variant.cc | 18 ++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/src/common/versioned_variant.h b/src/common/versioned_variant.h
index 9c9c5ada9b013..f7e46b7296e7f 100644
--- a/src/common/versioned_variant.h
+++ b/src/common/versioned_variant.h
@@ -16,6 +16,7 @@
 
 #include <concepts>
 #include <limits>
+#include <list>
 #include <variant>
 
 #include <boost/mp11/algorithm.hpp> // for mp_with_index
@@ -212,4 +213,22 @@ void decode(std::variant<Ts...>& v, bufferlist::const_iterator& p)
 
 } // namespace converted_variant
 
+
+/// \brief Generate a list with a default-constructed variant of each type.
+///
+/// This can be used in generate_test_instances() for types that contain
+/// variants to ensure that an encoding of each type is present in the
+/// ceph-object-corpus. This allows the ceph-dencoder tests to catch any
+/// breaking changes to the variant types that are present in encodings.
+template <typename ...Ts>
+void generate_test_instances(std::list<std::variant<Ts...>>& instances)
+{
+  // use an immediately-invoked lambda to get a parameter pack of variant indices
+  [&instances] <std::size_t ...I> (std::index_sequence<I...>) {
+    // use a fold expression to call emplace_back() for each index in the pack
+    // use in_place_index to default-construct a variant of the type at index I
+    (instances.emplace_back(std::in_place_index<I>), ...);
+  } (std::make_index_sequence<sizeof...(Ts)>{});
+}
+
 } // namespace ceph
diff --git a/src/test/common/test_versioned_variant.cc b/src/test/common/test_versioned_variant.cc
index b91e24b6f13fe..81f12c23c2b4f 100644
--- a/src/test/common/test_versioned_variant.cc
+++ b/src/test/common/test_versioned_variant.cc
@@ -13,6 +13,7 @@
  */
 
 #include "common/versioned_variant.h"
+#include <bitset>
 #include <string>
 #include <gtest/gtest.h>
 
@@ -320,4 +321,21 @@ TEST(ConvertedVariant, EncodeNew)
   }
 }
 
+TEST(Variant, GenerateTestInstances)
+{
+  using Variant = std::variant<int, bool, double>;
+
+  std::bitset<std::variant_size_v<Variant>> bits;
+  ASSERT_TRUE(bits.none());
+
+  std::list<Variant> instances;
+  generate_test_instances(instances);
+
+  for (const auto& v : instances) {
+    bits.set(v.index());
+  }
+
+  EXPECT_TRUE(bits.all());
+}
+
 } // namespace ceph
-- 
2.39.5