GCC 16.1 has been released! Lots of good C++26 material including reflection and contracts.

GCC 16 Release Series: Changes, New Features, and Fixes

From the announcement:

• C++20 by default: [...] N.B. C++20 modules support is still experimental and must be enabled by -fmodules.

• Several C++26 features have been implemented:
  • P2996R13, Reflection (PR120775, enabled by -std=c++26 -freflection)
  • P3394R4, Annotations for Reflection
  • P3293R3, Splicing a base class subobject
  • P3096R12, Function Parameter Reflection
  • P3491R3, define_static_{string,object,array} (PR120783)
  • P3560R2, Error Handling in Reflection
  • P1306R5, Expansion statements (PR120776)
  • P2900R14, Contracts (PR119061)
  • P2795R5, Erroneous behavior for uninitialized reads (PR114457)
  • [...]

• Improved experimental C++20 modules support:
  • New command line option --compile-std-module that conveniently builds the <bits/stdc++.h> header unit and the std and std.compat modules before compiling any source files explicitly specified on the command line.
  • Whenever the <bits/stdc++.h> header unit has been built, GCC now transparently translates an #include of any importable standard library header into an import of <bits/stdc++.h>.
  • Many reported bugs have been fixed, thanks to Nathaniel Shead.
  • [...]

Runtime Library (libstdc++)

• [...]
• Improved experimental support for [...]
  • std::mdspan, thanks to Luc Grosheintz.
  • [...]
  • std::simd.
  • std::inplace_vector.
  • [...]