YQL-21408: Make TFeature constexpr

#### Make TFeature constexpr and enhance langversion validation ✎

- Refactored `TFeature` to be fully `constexpr` by removing the `Finish()` method and introducing a `Checked()` static method for compile-time validation of language version ranges.
- Updated feature generation logic to initialize features as `inline constexpr` with direct validation during compilation.
- Moved `IsValidLangVersion` implementation to header file and made it `constexpr` for compile-time checks.
- Added new feature `Interval64Seconds` with minimum language version `2025.03`.
- Updated code to use the new `Interval64Seconds` feature for conditional compilation instead of hardcoded version numbers.

<a href="https://nda.ya.ru/t/qa0kX64r7DqvtN"><font size="2">Autodescription by Yandex Code Assistant</font></a>
commit_hash:504d387ebb1cc3958ee0e916cfc3e83691c756fe

7 files changed, 32 insertions, 30 deletions

diff --git a/yql/essentials/core/langver/feature.cpp b/yql/essentials/core/langver/feature.cpp
index db9b7659e8b..4224a64036c 100644
--- a/yql/essentials/core/langver/feature.cpp
+++ b/yql/essentials/core/langver/feature.cpp
@@ -45,16 +45,6 @@ std::expected<std::monostate, TError> GetAvailability(
 
 } // namespace
 
-TFeature TFeature::Finish() && {
-    YQL_ENSURE(
-        MinLangVer == UnknownLangVersion || IsValidLangVersion(MinLangVer),
-        "LangVer: " << FormatLangVersion(MinLangVer));
-    YQL_ENSURE(
-        MaxLangVer == UnknownLangVersion || IsValidLangVersion(MaxLangVer),
-        "LangVer: " << FormatLangVersion(MaxLangVer));
-    return *this;
-}
-
 bool IsAvailableOn(
     TLangVersion current,
     EBackportCompatibleFeaturesMode mode,
diff --git a/yql/essentials/core/langver/feature.gen.py b/yql/essentials/core/langver/feature.gen.py
index 4f1fa5ba585..ece5a8a75bc 100644
--- a/yql/essentials/core/langver/feature.gen.py
+++ b/yql/essentials/core/langver/feature.gen.py
@@ -54,12 +54,12 @@ def emit_feature(name: str, attrs: dict[str, Any]) -> Generator[str]:
     min_langver = parse_langver(attrs.get("min_langver", "unknown"))
     max_langver = parse_langver(attrs.get("max_langver", "unknown"))
 
-    yield f'const TFeature {name} = (TFeature{{'
+    yield f'inline constexpr TFeature {name} = TFeature::Checked({{'
     yield f'    .Name = "{name}",'
     yield f'    .Description = "{description}",'
     yield f'    .MinLangVer = {min_langver},'
     yield f'    .MaxLangVer = {max_langver},'
-    yield f'}}).Finish();'
+    yield f'}});'
     yield ""
 
 
diff --git a/yql/essentials/core/langver/feature.h b/yql/essentials/core/langver/feature.h
index 48a6c9d5ad1..a97df4a9634 100644
--- a/yql/essentials/core/langver/feature.h
+++ b/yql/essentials/core/langver/feature.h
@@ -7,12 +7,22 @@
 namespace NYql {
 
 struct TFeature {
-    TString Name;
-    TString Description;
-    TLangVersion MinLangVer = UnknownLangVersion;
-    TLangVersion MaxLangVer = UnknownLangVersion;
+    const TStringBuf Name;
+    const TStringBuf Description;
+    const TLangVersion MinLangVer = UnknownLangVersion;
+    const TLangVersion MaxLangVer = UnknownLangVersion;
 
-    TFeature Finish() &&;
+    static consteval TFeature Checked(TFeature f) {
+        if (f.MinLangVer != UnknownLangVersion && !IsValidLangVersion(f.MinLangVer)) {
+            throw std::invalid_argument("Bad MinLangVer");
+        }
+
+        if (f.MaxLangVer != UnknownLangVersion && !IsValidLangVersion(f.MaxLangVer)) {
+            throw std::invalid_argument("Bad MaxLangVer");
+        }
+
+        return f;
+    }
 };
 
 bool IsAvailableOn(
diff --git a/yql/essentials/data/language/features.json b/yql/essentials/data/language/features.json
index 1cb27b27cb6..d96fd7ad201 100644
--- a/yql/essentials/data/language/features.json
+++ b/yql/essentials/data/language/features.json
@@ -147,6 +147,9 @@
     "ValidateRegexp": {
         "min_langver": "2025.03"
     },
+    "Interval64Seconds": {
+        "min_langver": "2025.03"
+    },
     "GroupByExprAfterWhere": {
         "min_langver": "2025.02"
     },
diff --git a/yql/essentials/public/langver/yql_langver.cpp b/yql/essentials/public/langver/yql_langver.cpp
index 042584e5cc6..a7f5a6d79da 100644
--- a/yql/essentials/public/langver/yql_langver.cpp
+++ b/yql/essentials/public/langver/yql_langver.cpp
@@ -13,16 +13,6 @@ constexpr TLangVersion MaxReleasedLangVersion = MakeLangVersion(2026, 1);
 
 } // namespace
 
-bool IsValidLangVersion(TLangVersion ver) {
-    for (const auto& version : Versions) {
-        if (ver == MakeLangVersion(version.first, version.second)) {
-            return true;
-        }
-    }
-
-    return false;
-}
-
 bool ParseLangVersion(TStringBuf str, TLangVersion& result) {
     result = UnknownLangVersion;
     if (str.size() != 7 || str[4] != '.') {
diff --git a/yql/essentials/public/langver/yql_langver.h b/yql/essentials/public/langver/yql_langver.h
index f9631c88b72..939cde3e5e8 100644
--- a/yql/essentials/public/langver/yql_langver.h
+++ b/yql/essentials/public/langver/yql_langver.h
@@ -54,6 +54,16 @@ constexpr inline bool IsUnsupportedLangVersion(TLangVersion ver, TLangVersion ma
     return GetYearFromLangVersion(ver) <= GetYearFromLangVersion(max) - 3;
 }
 
+constexpr bool IsValidLangVersion(TLangVersion ver) {
+    for (const auto& version : Versions) {
+        if (ver == MakeLangVersion(version.first, version.second)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 constexpr TLangVersion MinLangVersion = MakeLangVersion(2025, 1);
 
 TLangVersion GetMaxReleasedLangVersion();
@@ -71,7 +81,6 @@ constexpr TLangVersion GetMaxLangVersion() {
 constexpr ui32 LangVersionBufferSize = 4 + 1 + 2 + 1; // year.minor\0
 using TLangVersionBuffer = std::array<char, LangVersionBufferSize>;
 
-bool IsValidLangVersion(TLangVersion ver);
 bool ParseLangVersion(TStringBuf str, TLangVersion& result);
 bool FormatLangVersion(TLangVersion ver, TLangVersionBuffer& buffer, TStringBuf& result);
 TMaybe<TString> FormatLangVersion(TLangVersion ver);
diff --git a/yql/essentials/udfs/common/datetime2/datetime_udf.cpp b/yql/essentials/udfs/common/datetime2/datetime_udf.cpp
index 50b70cc59d8..8841743ebb1 100644
--- a/yql/essentials/udfs/common/datetime2/datetime_udf.cpp
+++ b/yql/essentials/udfs/common/datetime2/datetime_udf.cpp
@@ -3752,7 +3752,7 @@ SIMPLE_MODULE(TDateTime2Module,
               TIntervalFromMinutes,
 
               TLangVerForked<
-                  NYql::MakeLangVersion(2025, 03), // TODO(YQL-21408)
+                  NYql::NFeature::Interval64Seconds.MinLangVer,
                   NLegacy::TIntervalFromSeconds,
                   NActual::TIntervalFromSeconds>,
 
@@ -3802,7 +3802,7 @@ SIMPLE_MODULE(TDateTime2Module,
                                   SimpleDatetimeToIntervalUdf<TM64ResourceName, EndOf<TTM64Storage>>>,
 
               TLangVerForked<
-                  NYql::MakeLangVersion(2025, 03), // TODO(YQL-21408)
+                  NYql::NFeature::Interval64Seconds.MinLangVer,
                   TToUnits<ToSecondsUDF, /* TResult = */ ui32, /* TSignedResult = */ i32, /* TWResult = */ i64, 1>,
                   TToUnits<ToSecondsUDF, /* TResult = */ ui32, /* TSignedResult = */ i64, /* TWResult = */ i64, 1>>,