https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65396
Jonathan Wakely <redi at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Last reconfirmed|2016-01-24 00:00:00 |2019-1-16 --- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> --- From PR 88864: Shorter repro from StackOverflow https://stackoverflow.com/q/54202462/2069064: struct B { template<typename T, typename U> B(T t); }; template <typename T, typename U = int> B::B(T t) { } B b(3); This is rejected by all versions of gcc because of an inability to deduce U.