The following code (reduced from a custom std::variant-like type) compiles with GCC 12.2 and Clang, but is rejected by GCC 13+: ``` template <typename T, typename> struct VariantConstructors { VariantConstructors(T&& t) requires(requires { T(t); }); }; template <typename... Ts> struct InheritFromEntries : Ts... {}; template <typename... Ps> struct InheritFromPack: InheritFromEntries<Ps>... { using InheritFromEntries<Ps>::InheritFromEntries...; }; template <typename... Ts> struct Variant : InheritFromPack<VariantConstructors<Ts, Variant<Ts...>>...> {}; template <typename T> class Outer; struct Inner { Inner(Outer<int>); }; template<typename T> class Outer { Variant<T, Inner> value_; }; struct Empty {}; void fn(Outer<Empty> x) {} ``` The following compiler error is produced (arguments: -std=c++20): <source>: In instantiation of 'VariantConstructors<T, <template-parameter-1-2> >::VariantConstructors(T&&) requires requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);} [with T = Inner; <template-parameter-1-2> = Variant<int, Inner>]': <source>:12:51: required from 'struct InheritFromPack<VariantConstructors<int, Variant<int, Inner> >, VariantConstructors<Inner, Variant<int, Inner> > >' <source>:16:8: required from 'struct Variant<int, Inner>' <source>:25:23: required from 'class Outer<int>' <source>:4:25: required from 'VariantConstructors<T, <template-parameter-1-2> >::VariantConstructors(T&&) requires requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);} [with T = Inner; <template-parameter-1-2> = Variant<Empty, Inner>]' <source>:12:51: required from 'struct InheritFromPack<VariantConstructors<Empty, Variant<Empty, Inner> >, VariantConstructors<Inner, Variant<Empty, Inner> > >' <source>:16:8: required from 'struct Variant<Empty, Inner>' <source>:25:23: required from 'class Outer<Empty>' <source>:29:23: required from here <source>:3:3: required by the constraints of 'template<class T, class> VariantConstructors<T, <template-parameter-1-2> >::VariantConstructors(T&&) requires requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);}' <source>:4:14: in requirements [with T = Inner] <source>:4:14: error: satisfaction of atomic constraint 'requires{T(VariantConstructors<T, <template-parameter-1-2> >::__ct ::t);} [with T = T]' depends on itself 4 | requires(requires { T(t); }); | ~^~~~~~~~~~~~~~~~~~~ Reproducer on Compiler Explorer: https://godbolt.org/z/TbcoanG5T The non-reduced preprocessed source can be found here: https://godbolt.org/z/69dMMoWKh
As a note, using a type alias for `T` in the constraint seems to work around the issue: ``` template <typename T, typename> struct VariantConstructors { using U = T; VariantConstructors(T&& t) requires(requires { U(t); }); }; // Everything below is the same as the repro case template <typename... Ts> struct InheritFromEntries : Ts... {}; template <typename... Ps> struct InheritFromPack: InheritFromEntries<Ps>... { using InheritFromEntries<Ps>::InheritFromEntries...; }; template <typename... Ts> struct Variant : InheritFromPack<VariantConstructors<Ts, Variant<Ts...>>...> {}; template <typename T> class Outer; struct Inner { Inner(Outer<int>); }; template<typename T> class Outer { Variant<T, Inner> value_; }; struct Empty {}; void fn(Outer<Empty> x) {} ``` (https://godbolt.org/z/1GTrjcvG6)
Thanks for the bug report. Started with r13-980-gdf4f95dbd4764f which made function parameters used in a constraint no longer induce a dependency on all contextual template parameters. So another workaround would be to make the constructor's constraints explicitly depend on the second template parameter of the class: template <typename T, typename U> struct VariantConstructors { VariantConstructors(T&& t) requires(requires { T(t); typename U; }); }; ...
GCC 13.2 is being released, retargeting bugs to GCC 13.3.
GCC 13.3 is being released, retargeting bugs to GCC 13.4.