https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109247
--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> --- No templates (so it's ambiguous, but the problem is the same): struct B { int a; }; struct C { explicit C (int); C& operator= (B); C& operator= (const C&); }; void f(C x) { x = { 42 }; }