https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99423
--- Comment #1 from Nathan Sidwell <nathan at gcc dot gnu.org> --- // bug_a.ii template<typename _T1> struct pair { pair() { } }; inline pair<bool> blob () { return {}; } // bug_b.ii import "bug_a.ii"; pair<bool> boom(); ./cc1plus -fmodule-header=system -std=c++20 bug_a.ii -fpreprocessed -quiet && ./cc1plus -fmodule-header=system -std=c++20 bug_b.ii -fpreprocessed -quiet --param ggc-min-expand=0 --param ggc-min-heapsize=0 bug_b.ii:4:10: internal compiler error: tree check: expected tree_vec, have ggc_freed in lookup_template_class_1, at cp/pt.c:9803 4 | pair<bool> boom(); | ^ 0x1acdc4b tree_check_failed(tree_node const*, char const*, int, char const*, ...) ../../../src/gcc/tree.c:9814 0xad685c tree_check(tree_node*, char const*, int, char const*, tree_code) ../../../src/gcc/tree.h:3353 0xdc6b60 lookup_template_class_1 ../../../src/gcc/cp/pt.c:9803