https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93467
Bug ID: 93467 Summary: [concepts] getting "type constraint differs in template redeclaration" error after friend declaration in template Product: gcc Version: 9.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: robert.zavalczki at gmail dot com Target Milestone: --- I know the C++ standard says: > "A constrained declaration may only be redeclared using the same syntactic > form. No diagnostic is required." However, that specification causes this supposedly correct program to fail: /////////////////////////////////////// // compile with: g++-9 -fconcepts -std=c++2a template<typename T> concept True = true; template<typename U> struct S1 { template<True T> friend struct S2; // friend declaration for S2 }; S1<int> s; // instantiate S1 template<True T> struct S2; // another declaration for S2 /////////////////////////////////////// You can test the above snippet on godbolt.org: https://godbolt.org/z/V6uJPe