https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99699
Bug ID: 99699 Summary: Type deduction failure for deducing a non-type template parameter via another deducible structural type (class template specialization) non-type template parameter Product: gcc Version: 11.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: davveston at gmail dot com Target Milestone: --- The following program: // -------------------------------------------- template<int N> struct S {}; template<S s> struct U {}; template<typename> struct V { V() = delete; }; template <int N, S<N> s> struct V<U<s>> {}; V<U<S<0>{}>> v{}; // #1 // Expected: use partial specialization #1 // GCC actual: error (rejects-valid): use of deleted function int main() {} // -------------------------------------------- is rejected by gcc HEAD 11.0.1 20210320 (experimental) when compiling with 'g++ prog.cc -Wall -Wextra -std=c++2b', as #1 matches to the primary template instead of the partial specialization. It seems to find the non-type template parameter N non-deducible; however all arguments of the partial specialization are arguably deducible from the 'U<S<0>{}>' argument to the primary template: - 'A' / 'P' matching 'U<S<0>{}>' / 'U<s>' deduces 's' to 'S<0>{}' and its type to 'S<0>', - 'A' / 'P' matching 'S<0>' to 'S<N>' deduces 'N' to '0'; 'N' is particularly not in a non-deduced context above, as it's not used in sub-expression in the non-type template parameter 's' of the partial specialization. Demo: https://wandbox.org/permlink/QQQxa3BhBTMM476E