GCC 16.1 internal compiler error in add_imported_namespace when mcpp.build.prepare gains an interface unit in its import chain #721

Observation

GCC 16.1 (xim gcc@16.1.0, the toolchain mcpp's manifest pins on Linux) segfaults while compiling src/main.cpp whenever mcpp.build.prepare gains a new interface unit in its import chain:

src/main.cpp:5:17: internal compiler error: Segmentation fault
    5 | import mcpp.cli;

The backtrace reported during the first occurrence runs through module_state::read_namespaces → add_imported_namespace → get_originating_module_decl. Every module unit compiles; only the import in main.cpp fails. LLVM 22.1.8 builds the same trees.

Measurements

Each row is a full build of mcpp on x86_64 Linux.

The trigger is structural: a new interface unit (a named module or an interface partition) between mcpp.build.prepare and its imports crashes the compiler whatever the unit contains (E4). Implementation units, and an implementation partition that only implementation units import, stay out of the interface's dependency chain and compile (E5, E6).

Earlier observations with the same signature:

• the cloud session that produced 2026.9.27.1 could not remove any of the 22 top-level declarations of src/cli/cmd_build.cppm while keeping the crash, and found it needs an import chain of depth two (main imports A, A imports B, B imports mcpp.cli.cmd_build);
• src/project.cppm carries a copy of package_key because importing mcpp.xlings.address_set there crashed the same way (see the comment near inherit_workspace_xlings).

Consequence for mcpp

src/build/prepare.cppm is decomposed in 2026.9.27.1 (#719) using only implementation units and an implementation partition imported by implementation units, per E5 and E6. A future change that adds an interface partition or a new named module to that chain will hit this crash on GCC 16.1.

Next steps

• Reduce outside mcpp to a standalone reproducer and report it to GCC bugzilla (needs an account).
• Re-test with a GCC 16 branch snapshot when xim carries one.