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Pull Request Overview
This PR upgrades the codebase to require C++20 as the minimum standard for extension builds. The change modernizes the codebase by replacing older C++17 constructs and string formatting approaches with C++20 features.
Key changes:
- Updates minimum C++ standard requirement from C++17 to C++20
- Replaces
std::ostringstreamstring formatting with C++20std::format - Modernizes template syntax using C++20
requiresclauses instead of SFINAE - Updates build configuration and CI workflows to use C++20
Reviewed Changes
Copilot reviewed 20 out of 20 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| CMakeLists.txt | Updates minimum C++ standard requirement from 17 to 20 |
| src/treespec/unflatten.cpp | Replaces ostringstream with std::format, adds std::span usage |
| src/treespec/treespec.cpp | Extensive modernization with std::format, std::span, std::ranges, and constinit |
| src/treespec/traversal.cpp | Updates string formatting to use std::format |
| src/treespec/serialization.cpp | Uses std::unordered_map::contains instead of find comparison |
| src/treespec/richcomparison.cpp | Replaces std algorithm calls with std::ranges equivalents |
| src/treespec/hashing.cpp | Uses std::unordered_map::contains for cleaner code |
| src/treespec/flatten.cpp | Comprehensive string formatting updates and template parameter renaming |
| src/treespec/constructors.cpp | String formatting modernization and std::ranges usage |
| src/registry.cpp | Updates string formatting and replaces PYBIND11_CONSTINIT with constinit |
| src/optree.cpp | String formatting updates and constinit usage |
| include/optree/treespec.h | Updates function signatures to use std::span and modernizes template parameters |
| include/optree/pytypes.h | Adds std::formatter specializations and modernizes template syntax with requires clauses |
| include/optree/pymacros.h | Replaces PYBIND11_CONSTINIT with constinit |
| include/optree/exceptions.h | Major refactoring using std::source_location and std::format for error handling |
| README.md | Updates documentation to reflect C++20 requirement |
| CPPLINT.cfg | Adds whitespace/braces filter |
| .github/workflows/tests.yml | Removes C++17 compatibility test |
| .github/workflows/lint.yml | Updates clang-tidy to use C++20 |
| .clang-format | Updates standard from c++17 to c++20 |
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
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Files 15 15
Lines 1623 1623
Branches 218 218
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Code reviewNo issues found. Checked for bugs and CLAUDE.md compliance. 🤖 Generated with Claude Code - If this code review was useful, please react with 👍. Otherwise, react with 👎. |
r-barnes
approved these changes
Mar 8, 2026
Merge the correctness-audit batch from `main` into the C++20 modernization branch. The two sides are orthogonal in intent but heavily co-located in text: `main` fixes semantics (lock ordering, GC reporting, namespace-merge safety, pickle validation), while `cpp20` rewrites the same lines into C++20 idioms (`std::format`, `std::span`, `std::source_location`, `std::ranges`, `requires`, `.contains()`). Every conflict is therefore resolved as *their* logic expressed in *our* idiom, never by picking a side. Explicit conflicts (18 hunks across 9 files): * `exceptions.h`: our include set plus their `<utility>` for `std::declval`. * `pytypes.h`: `[[nodiscard]]` combined with `requires` on `TupleGetItemAs`, `ListGetItemAs` and `DictGetItemAs`; their copy-then-commit `TotalOrderSort` with our `std::format` sort key; `MAX_TYPE_CACHE_SIZE` moved beside their `WeakKeyCache`. * `registry.h`: their `DictInsertionOrderedFlags` snapshot with `.contains()`. * `treespec.h`: our `std::span<py::object>` `MakeNode`; their C-array NOLINT re-tag no longer applies. * `registry.cpp`: their `RegistryStatus` restructure taken wholesale, with our `std::format` messages re-applied at the new post-unlock site. * `flatten.cpp`: their `leaves.size() != num_leaves` condition, our message. * `richcomparison.cpp`, `traversal.cpp`: unioned includes. * `treespec.cpp`: their `sorted_other_keys` snapshot and recursion `depth` argument, our `std::format` and `std::ranges::reverse`. Implicit conflicts (clean merges that split the codebase's idiom): * Converted the five single-shot messages `main` added as `std::ostringstream` to `std::format`, and its four new `find() != end()` to `.contains()` and one `enable_if_t` to `requires`. Branchy message builders stay `ostringstream`, matching the convention on both sides. * `serialization.cpp` used `std::format` without including `<format>`, and its `malformed` lambda still concatenated with `+`. * `RelpathFromProjectRoot()` is now total: a translation unit whose path is shorter than this header's would have thrown `std::out_of_range` out of `PyInit__C`. * Dropped four `NOLINT[...pointer-arithmetic]` comments made dead by `std::span::operator[]`. Keeping the two type-fixing `std::formatter` specializations is load-bearing. Collapsing them into one constrained on `std::is_base_of_v<py::handle, T>` gives the same template argument list as the standard range formatter, and since neither constraint subsumes the other, every pybind11 wrapper becomes ambiguous from C++23 on. That regression was introduced during this merge, caught by review, and reverted; all 11 translation units now compile clean at both C++20 and C++23 on libc++ and libstdc++. Swept the tree for C++20 features neither branch had applied, rather than only checking that each branch's own changes survived: * `std::memory_order_acquire` / `_release` -> `std::memory_order::acquire` / `::release`. C++20 made `std::memory_order` a scoped enum (P0439R0) and keeps the unscoped names only as compatibility constants. These arrived with the new `PyTreeIter` re-entrancy guard from `main`, so neither branch had converted them. * `has_to_string` was a `std::void_t` detection idiom built from a partial specialization over `std::true_type` / `std::false_type`; it is now a concept. Constraining on `const T &` also matches how `try_to_string` actually calls `std::to_string`, where the old form probed an rvalue. `exceptions.h` no longer needs `<type_traits>` or `<utility>`. * `std::cmp_less` (P0586R2) replaces a `static_cast<ssize_t>` around a signed/unsigned comparison in `FromPicklable`. * One residual idiom split: `flatten.cpp`'s dictionary key mismatch still built its message with `ostringstream` while the identical message in `treespec.cpp` had been converted. It now uses `std::format` with `NodeKindToString()`, which already is the single source of truth for those three kind names. Neither of the first two is reported by clang-tidy: there is no check for the scoped `memory_order` enumerators, and `modernize-use-constraints` only rewrites `std::enable_if`, not `void_t` detection. The rest of the surface is clean: no non-ranges algorithms, erase-remove idioms, `compare(0, n, ...)` prefix tests, C arrays, `typedef`, `std::bind` or raw pointer-and-size parameter pairs remain. Give the registry's pair keys transparent hashers and comparators whose call operators are templates, and declare the two containers with them. An `is_transparent` marker only tells the container it may forward a foreign key type; a non-template call operator then converts that key back to the exact `key_type`, which is the very temporary heterogeneous lookup exists to avoid. The marker was inert for unordered containers before P0919R3, so this cost nothing until the standard moved: measured on a standalone model of `PyTreeTypeRegistry::Lookup`, identical source allocates once per `find` at `-std=c++17` and three times at `-std=c++20`. Probing with `std::string_view` takes it to zero, worth -22.9% on flattening a tree of namespaced custom nodes (6063 -> 4676 ns/op; the penalty over a global-namespace flatten drops from +53% to +22%). This is pre-existing on `main`, which already defaults to C++20; it is included here because the branch makes that default unconditional. Also folded in, from reviewing the C++17 -> C++20 bump as a whole: * Set `MACOSX_DEPLOYMENT_TARGET` to 14.0. Apple's libc++ gates `std::format` behind macOS 13.3, but PEP 425 floors the macOS tag's minor version to zero, so a 13.3 binary ships as `macosx_13_0_*`, installs on 13.0 through 13.2 and then fails in `dyld`. 14.0 is the lowest target whose tag does not lie. The previous 15.0 was safe but dropped macOS 14 needlessly. * Retag the iOS wheels to match `IPHONEOS_DEPLOYMENT_TARGET`. cibuildwheel's cross-build environment reports a hardcoded `ios-13.0-*` platform, so the wheels shipped as `ios_13_0_*` while the binary required 16.3; `pip` accepted them on iOS 13.0 through 16.2 and `dyld` then failed to load `_C`. Unlike macOS, iOS tags carry the minor version, so 16.3 is expressible exactly. * Stop passing `-Wno-error=attributes`. It existed so `[[likely]]` and `[[unlikely]]` would not fail a C++17 `-Werror` build and now only masks genuine attribute errors. * Record the breaking standard bump in `CHANGELOG.md` and give `README.md` the concrete compiler floors: GCC 13+, Clang 16+ with libstdc++ 13+, Clang 17+ with libc++ 17+, Apple Clang 16+ (Xcode 16+), or MSVC 19.32+ (VS 2022 17.2+). `std::format` binds on GCC and MSVC; on Clang it is P0634R3. MSVC 19.30 and 19.31 cannot build this at `/std:c++20` at all, since MS STL gated `__cpp_lib_format` behind `_HAS_CXX23` until 17.2, so the floor is not the interval "19.29 and later". Tests: cover the messages this merge rewrote, which had none. Pin the full interpolated text of the namedtuple `_fields` arity mismatch (a swapped placeholder previously matched), the `Malformed pickled PyTreeSpec` template, and the stale-custom-registration rejection from `treespec_from_collection()`. Add `MakeNode`'s empty-`std::span` path across every arity-0 container, including one nested among non-empty siblings. Verified: 94205 passed, 1 skipped, 1 xfailed; clean build at `OPTREE_CXX_WERROR=ON`; all pre-commit hooks pass; every translation unit compiles clean at both C++20 and C++23 under Clang/libc++ and builds and links under GCC 14/libstdc++. clang-tidy reports nothing across all 20 files, run under `quay.io/pypa/manylinux_2_28` because the check set needs a toolchain whose libstdc++ can resolve the C++20 headers -- though note that the project's `NOLINT[...]` bracket syntax is not parsed as a check list by clang-tidy or cpplint, so each marker suppresses every check on its line rather than the ones it names.
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Description
Raise the minimum C++ standard from C++17 to C++20 and adopt C++20 features throughout the extension codebase.
C++20 features adopted
std::format: Replace allstd::ostringstreamandoperator+string concatenation for error/exception messages withstd::format. Addstd::formatterspecializations forpy::handleandpy::object.std::source_location: Replace__FILE__,__LINE__, and__PRETTY_FUNCTION__macros inInternalErrorwithstd::source_location::current()default parameters.std::span: Replace raw pointer + size pairs (py::object *children, ssize_t num_children) withstd::span<py::object>inMakeNodeand related functions.std::ranges: Replacestd::copy/std::reversewith iterator pairs bystd::ranges::copy/std::ranges::reversewith range arguments.requiresclauses: Replacestd::enable_if_tSFINAE with C++20requiresexpressions in template functions (TupleGetItemAs,ListGetItemAs,DictGetItemAs).std::unordered_map::contains: Replace.find() != .end()pattern with.contains().Build and CI changes
CMakeLists.txt: Raise minimum standard from C++17 to C++20.MACOSX_DEPLOYMENT_TARGET=15.0andIPHONEOS_DEPLOYMENT_TARGET=16.3(required forstd::formatsupport in Apple libc++)..clang-formatstandard toc++20andclang-tidyto use C++20.Motivation and Context
C++20 provides standard library features that simplify the codebase and improve code clarity:
std::formateliminates verbosestd::ostringstreampatterns for string formatting.std::source_locationreplaces non-portable compiler macros (__PRETTY_FUNCTION__) with a standard mechanism.std::spanprovides a safer, more expressive alternative to raw pointer + size parameters.This is a breaking change: C++20 compiler support is now required to build the extension from source.
Types of changes
Checklist
make format. (required)make lint. (required)make testpass. (required)