Add YT_DEFINE_SENTINEL_OPTIONAL macro

Add a macro for defining `TSentinelOptional` type aliases when the value type is not structural (e.g. has protected members) and therefore cannot use `TValueSentinel<V>`. The macro collapses the boilerplate sentinel struct plus `using` declaration into a single line:

```cpp
// before
struct TInstantSentinel
{
    static constexpr auto Sentinel = TInstant::Zero();
};
using TSentinelOptionalInstant = TSentinelOptional<TInstant, TInstantSentinel>;

// after
YT_DEFINE_SENTINEL_OPTIONAL(TSentinelOptionalInstant, TInstant, TInstant::Zero());
```

Also convert the existing call sites in `service_detail.cpp` and `inferrum/block_cache.cpp`.
commit_hash:5dcdeb8db215736b0ce5a5b71f30aead91c7b8e8

2 files changed, 17 insertions, 3 deletions

diff --git a/library/cpp/yt/containers/sentinel_optional.h b/library/cpp/yt/containers/sentinel_optional.h
index 1d573550db9..6178327e29f 100644
--- a/library/cpp/yt/containers/sentinel_optional.h
+++ b/library/cpp/yt/containers/sentinel_optional.h
@@ -109,6 +109,20 @@ private:
 
 } // namespace NYT
 
+//! Defines a type alias for |TSentinelOptional<type, ...>| with an inline sentinel struct.
+/*!
+ *  Example:
+ *    YT_DEFINE_SENTINEL_OPTIONAL(TInstantOpt, TInstant, TInstant::Zero());
+ */
+#define YT_DEFINE_SENTINEL_OPTIONAL(name, type, sentinel_value)  \
+    struct name##Sentinel                                        \
+    {                                                            \
+        static constexpr auto Sentinel = (sentinel_value);       \
+    };                                                           \
+    using name = ::NYT::TSentinelOptional<type, name##Sentinel>
+
+////////////////////////////////////////////////////////////////////////////////
+
 #define SENTINEL_OPTIONAL_INL_H_
 #include "sentinel_optional-inl.h"
 #undef SENTINEL_OPTIONAL_INL_H_
diff --git a/library/cpp/yt/containers/unittests/sentinel_optional_ut.cpp b/library/cpp/yt/containers/unittests/sentinel_optional_ut.cpp
index 64ec5a04367..3bce427555a 100644
--- a/library/cpp/yt/containers/unittests/sentinel_optional_ut.cpp
+++ b/library/cpp/yt/containers/unittests/sentinel_optional_ut.cpp
@@ -10,7 +10,7 @@ namespace {
 
 ////////////////////////////////////////////////////////////////////////////////
 
-using TIntOpt = TSentinelOptional<int, TValueSentinel<-1>>;
+YT_DEFINE_SENTINEL_OPTIONAL(TIntOpt, int, -1);
 
 ////////////////////////////////////////////////////////////////////////////////
 
@@ -153,7 +153,7 @@ TEST(TSentinelOptionalTest, ConstAccess)
 ////////////////////////////////////////////////////////////////////////////////
 // Pointer sentinel.
 
-using TPtrOpt = TSentinelOptional<int*, TValueSentinel<nullptr>>;
+YT_DEFINE_SENTINEL_OPTIONAL(TPtrOpt, int*, nullptr);
 
 TEST(TSentinelOptionalTest, PointerSentinel)
 {
@@ -178,7 +178,7 @@ enum class EColor
     Unknown,
 };
 
-using TColorOpt = TSentinelOptional<EColor, TValueSentinel<EColor::Unknown>>;
+YT_DEFINE_SENTINEL_OPTIONAL(TColorOpt, EColor, EColor::Unknown);
 
 TEST(TSentinelOptionalTest, EnumSentinel)
 {