https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115672
--- Comment #6 from Sean Murthy <s.murthy at outlook dot com> --- (In reply to Andrew Pinski from comment #4) > Created attachment 58528 [details] > Reduced further > > The second t argument type in the template definition is the key I think. > Changing it to just class allows the testcase to work. Yes, both observations I made in my follow up to 115656 apply here as well: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115656#c6 Those two observations give me the inkling the template template parameter and the template it references both being constrained is likely causing the issue: each of the observations call for only of those two to be constrained which causes the compiler to produce the expected output. Not surprisingly, the third point I made in that 115656 follow up also applies here: in the initial repro, use type_traits to constrain the class template dv, and that causes CTAD fail on the deducing ctor, regardless of if/how the template template parameter is constrained. This behavior suggests to me an issue with traits-concepts interplay/substitutability. And this behavior goes all the way back to GCC 10.1, making that particular aspect not a regression issue (unless something traits-specific pre-C++20 issue exists). A a1(45ul); //ctad fail if dv template constrained using traits See: https://sigcpp.godbolt.org/z/GqaPhG17W BTW thanks to all the GCC devs and maintainers for the awesome product and follow up. And please forgive me when I use incorrect C++ terminology. I'm just a 3/10 C++ programmer fumbling my way around. 🙏
