Clang 20.1.0 Release Notes - Clang 20.1.0 Documentation

From the release notes:

C++ Language Changes

• Allow single element access of GCC vector/ext_vector_type object to be constant expression. Supports the V.xyzw syntax and other tidbits as seen in OpenCL. Selecting multiple elements is left as a future work.
• Implement CWG1815. Support lifetime extension of temporary created by aggregate initialization using a default member initializer.
• Accept C++26 user-defined static_assert messages in C++11 as an extension.
• Add __builtin_elementwise_popcount builtin for integer types only.
• Add __builtin_elementwise_fmod builtin for floating point types only.
• Add __builtin_elementwise_minimum and __builtin_elementwise_maximum builtin for floating point types only.
• The builtin type alias __builtin_common_type has been added to improve the performance of std::common_type.

C++2c Feature Support

• Add __builtin_is_virtual_base_of intrinsic, which supports P2985R0 A type trait for detecting virtual base classes
• Implemented P2893R3 Variadic Friends
• Implemented P2747R2 constexpr placement new.
• Added the __builtin_is_within_lifetime builtin, which supports P2641R4 Checking if a union alternative is active
• Implemented P3176R1 The Oxford variadic comma

C++23 Feature Support

• Removed the restriction to literal types in constexpr functions in C++23 mode.
• Extend lifetime of temporaries in mem-default-init for P2718R0. Clang now fully supports P2718R0 Lifetime extension in range-based for loops.
• __cpp_explicit_this_parameter is now defined. (#82780)
• Add __builtin_is_implicit_lifetime intrinsic, which supports P2674R1 A trait for implicit lifetime types
• Add support for P2280R4 Using unknown pointers and references in constant expressions. (#63139)

C++20 Feature Support

• Implemented module level lookup for C++20 modules. (#90154)