https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121506
--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> --- DECL_ORIGINAL_TYPE should be set on typedefs (or C++ using), not on the actual struct/class/union type definitions. And the various va_list helper structures (with various __va_* names) are type definitions, not typedefs. And c_type_original that c_type_tag calls first should look through all the typedefs, so what c_type_tag sees afterward certainly has NULL DECL_ORIGINAL_TYPE.
