llvm

db291cbc - [SYCL] Make sub_group non-user-constructible in preview mode (#23042)

[SYCL] Make sub_group non-user-constructible in preview mode (#23042) SYCL 2020 section 4.9.1.8 specifies `sycl::sub_group` with a deleted default constructor, but the implementation declared it `protected` and defaulted instead. That is not equivalent: a constructor that is defaulted on its first declaration is not *user-provided*, so in C++17 `sub_group` remained an aggregate and `sycl::sub_group{}` was a well-formed aggregate initialization that bypassed the access check altogether: ```c++ // Compiles today with -std=c++17, which is the default: auto sg = sycl::sub_group{}; ``` This patch deletes the default constructor and adds a `protected` one taking a private tag. Being user-provided, it also makes the class a non-aggregate, which closes the C++17 loophole. `sub_group` objects are now created through that constructor in `nd_item::get_sub_group()` and `ext::oneapi::this_work_item::get_sub_group()`, both of which are already friends. Applications may rely on the current permissive behavior, so the change is guarded by `__INTEL_PREVIEW_BREAKING_CHANGES`. The deprecated `ext::oneapi::sub_group` also stops initializing its base through a private default constructor: copy-initializing the base works in both modes and needs no guard. ### Testing New `sycl/test/basic_tests/sub_group_interface.cpp` checks the SYCL 2020 synopsis, the common by-value semantics, and that the class can be constructed neither directly nor as an aggregate. It runs in four configurations: {C++17, C++20} x {default, `-fpreview-breaking-changes`}. Verified that legitimate ways of obtaining a sub-group still compile in all four configurations (`nd_item::get_sub_group()`, the free function queries, `khr::this_sub_group()`, copy/assign/compare, implicit conversion to the deprecated `ext::oneapi::sub_group`, group algorithms), and that `sycl::sub_group{}` still compiles outside of preview mode. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 5 <noreply@anthropic.com>