GCC 16.1 segfaults compiling src/main.cpp when the module graph gains one module #863

What happens

GCC 16.1.0 (the gcc@16.1.0 toolchain mcpp builds itself with) segfaults while compiling src/main.cpp (import mcpp.cli;) once the tree's module graph grows by one module. Measured while implementing the target sets of design 2026-10-29 (release/2026.10.30.1):

• adding a new module to the tree, whatever its content, makes the compile of src/main.cpp end with an internal compiler error (segmentation fault);
• the same happens with the unchanged src/cli/cmd_build.cppm plus one import of an existing, unrelated module (import mcpp.build.advice;);
• removing that one import makes the tree compile again.

The failure follows the number of modules reachable from main.cpp, not the code added, which points at GCC's handling of a large transitive module graph rather than at any construct of ours.

Workaround in mcpp

The target-set code, the leg API and the test archive were placed in existing modules (mcpp.toolchain.triple, mcpp.build.prepare with prepare/legs.cpp as a partition implementation unit, mcpp.build.execute) rather than in new ones, so the module count reachable from main.cpp is unchanged.

To do

Reduce to a self-contained reproducer (a generated chain of N modules imported by one TU) and report it to GCC Bugzilla; record the GCC bug here; remove the placement constraint when a fixed GCC is the toolchain mcpp builds with.

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