[VL] Fix Iceberg writer aggregate-init build failure on macOS C++20 libc++

[jackylee-ch]

What changes are proposed in this pull request?

The bug

Builds with --enable_enhanced_features=ON (which compiles the Iceberg
writer integration in cpp/velox/compute/iceberg/) fail to compile on
macOS Apple Clang 17 + libc++ with -std=c++20. The build aborts at
cpp/velox/compute/iceberg/IcebergWriter.cc.cpp.o (~156 / 217 in the
gluten cpp ninja graph) with:

error: no matching function for call to 'construct_at'
note: in instantiation of function template specialization
      'std::vector<IcebergPartitionSpec::Field>::emplace_back<...>'
candidate template ignored: substitution failure for construct_at
[with _Tp = IcebergPartitionSpec::Field, _Args = ...]:
no matching constructor for initialization of
'IcebergPartitionSpec::Field'

Root cause

IcebergWriter.cc constructs aggregate types with paren-init at
two call sites:

Line	Call	Target type	Origin
262	return WriteStats(bytes, files, ioNs, wallNs)	gluten::WriteStats	cpp/velox/compute/iceberg/IcebergWriter.h
311	fields.emplace_back(name, type, transform, parameter)	IcebergPartitionSpec::Field (Velox)	velox/connectors/hive/iceberg/PartitionSpec.h

Both target types are pure aggregate structs with no user-defined
constructor:

// Velox
struct IcebergPartitionSpec::Field {
  std::string name;
  TypePtr type;
  TransformType transformType;
  std::optional<int32_t> parameter;
};

// Gluten
struct WriteStats {
  uint64_t numWrittenBytes{0};
  uint32_t numWrittenFiles{0};
  uint64_t writeIOTimeNs{0};
  uint64_t writeWallNs{0};
  ...
};

C++20 P0960R3
made aggregates initializable via paren-init at the language level, but
standard library implementations of std::construct_at (which
std::vector::emplace_back, std::make_shared, std::optional::emplace
all route through for in-place construction) have not all caught up:

Toolchain	std::construct_at for aggregates	Build result
Linux GCC + libstdc++	Paren-init accepted (P0960 implemented)	✅ compiles
macOS Apple Clang 17 + libc++	Strict — only real constructors accepted	❌ substitution failure

So the same code that passes on Linux blocks the macOS build with
--enable_enhanced_features=ON.

The fix

Two minimal edits — switch paren-init to brace-init at the two call
sites:

- fields.emplace_back(name, type, transform, parameter);
+ fields.push_back({name, type, transform, parameter});

- return WriteStats(numWrittenBytes, numWrittenFiles, ioNs, wallNs);
+ return WriteStats{numWrittenBytes, numWrittenFiles, ioNs, wallNs};

Brace-init invokes aggregate initialization, which is supported on
every C++17/20 standard library. It does not depend on
std::construct_at accepting paren-init for aggregates, so the
result is portable.

Why this matters

1. Build blocker on macOS in enhanced mode. Anyone running
   dev/builddeps-veloxbe.sh --enable_enhanced_features=ON on macOS
   today hits a hard compile error before any binary is produced.
2. Naturally complements the macOS unblockers. Once the existing
   macOS build fixes land (gflags load-time abort, Arrow vendored
   datetime, /usr/local isolation), enhanced mode is the next thing
   macOS users will try — this PR removes the last compile-time
   blocker on that path.
3. Zero runtime impact. Brace-init and paren-init for aggregates
   produce identical field-by-field initialization on all
   toolchains. Linux libstdc++ still compiles cleanly. The change is
   purely build portability.
4. Style consistency. The rest of the iceberg/ directory and most
   of cpp/velox/ already uses brace-init for aggregate construction.
   These two paren-init call sites are the outliers.
5. Tiny diff — 2 hunks, 6 lines (3 added, 3 removed).

How was this patch tested?

• macOS 14 arm64 + Apple Clang 17 + libc++ + -std=c++20 +
  --enable_enhanced_features=ON:
  • Before fix: build aborts at IcebergWriter.cc.cpp.o
    (~156 / 217 ninja steps) with the construct_at substitution
    failure.
  • After fix: build reaches 217 / 217, including the new
    velox_iceberg_test executable produced under the enhanced-features
    gate.
• macOS cpp full ctest matrix (17 binaries including
  velox_iceberg_test): 5539 / 5539 pass with this fix.
• Linux x86_64 Ubuntu 22.04 build remains green; brace-init for
  aggregates is the same lowering as the prior paren-init under
  libstdc++'s P0960 implementation, so behavior is identical.

Was this patch authored or co-authored using generative AI tooling?

co-auth: Claude (Sonnet/Opus) via Claude Code 1.x