GeezerVison

0.20.0: a SYCL build has one C++ runtime, and it is libstdc++ (mcpp-community/mcpp#841, mcpp#838).

libsycl.so throws sycl::exception on libstdc++. A typed catch does not match across two C++ runtimes in one process, so 0.19's -l:libstdc++.so.6 beside an llvm build's libc++ let only catch (...) see what the runtime threw. Changes:

• mcpp.rules.sycl names no C++ library on the link line. On Linux it refuses a build whose C++ library (mcpp::cxx_stdlib()) is not libstdc++, before any unit is compiled, and names both remedies (gcc.libstdcxx beside llvm, or a gcc toolchain). --gcc-install-dir follows the build's own GCC, in this order: the graph's supplier (MCPP_CXX_ABI_ROOT, read from the environment so the rule still compiles on older engines), the gcc toolchain's payload, then the xim:gcc the rule declares.
• tests/sycl-consumer declares gcc.libstdcxx and the runtime adapter under cfg(accelerator = "sycl"), so the CPU variant (--no-accel) keeps llvm's libc++.
• Docs (docs/rules.md), version 0.20.0. CI runs mcpp 2026.10.26.1.

Measured locally with dpcpp 7.1.0:

• llvm + gcc.libstdcxx: builds and runs.
• gcc: builds and runs.
• llvm without a supplier: refused, with both remedies.
• --no-accel: builds and runs on libc++.

The SYCL consumer needs gcc.libstdcxx from mcpplibs/mcpp-index#568.

Co-authored-by: speak-agent [email protected]

Platform plugins and the package reader (mcpp#847)

• mcpp::plugins::package (in mcpp.plugins, std-only) reads a staged package's mcpp-package.json (mcpp SPEC-014 §4.5): read(stage_dir) returns std::expected<info, std::string>, and files_with_role and entry_of select from it. A schema other than 1 is refused.
• Feature plugins-platform adds module mcpp.plugins.platform, which states what a platform's plugin (SPEC-013 §12) and a package format's provider state:
  • link::driver, link::start, link::input_file, link::input_from, artifact_names, seal and run_host, each checked at the call;
  • provide_format(name, make), which declares the format on every pass and submits the artifact action only in the pass that asks for it, made from the staged package.
• mcpp.plugins.testing gains context::pack(format, stage_dir).
• tests/plugin-logic gains 8 cases; all 37 cases pass locally with mcpp built from release/2026.10.27.1.
• The package floor is now mcpp 2026.10.27.1. CI builds that engine from the repository variable MCPP_SOURCE_REF (release/2026.10.27.1) until the release exists. This covers the editor lane, whose mcpp moves to 2026.10.26.2 so it can build the engine under review.