https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71843
--- Comment #1 from Tom Honermann <tom at honermann dot net> --- Created attachment 38876 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38876&action=edit Patch to elucidate failed constraints The attached patch was provided to me by Andrew Sutton earlier this year and applies cleanly to gcc 6 trunk (r238201). The patch modifies the error messages emitted for constraint satisfaction failures so that the specific constraints that were not satisfied will be listed, up to a maximum of 7 constraints for each overload candidate. Ideally, the number of constraints to list would be controlled by command line options, but the patch does not currently provide such control. Example diagnostics provided by this patch: $ cat t.cpp template<typename T> concept bool C = requires(T t) { typename T::type; t.i; t.i2; t.i3; t.i4; t.i5; t.i6; t.i7; t.i8; }; template<C T> auto f(T t) { return t.i; } struct S { }; auto x = f(S{}); $ g++ -c -std=c++1z -fconcepts t.cpp t.cpp:19:15: error: cannot call function ‘auto f(T) [with T = S]’ auto x = f(S{}); ^ t.cpp:14:6: note: constraints not satisfied auto f(T t) { ^ t.cpp:14:6: note: concept ‘C<S>’ was not satisfied t.cpp:2:14: note: within the concept template<class T> constexpr const bool C<T> [with T = S] concept bool C = requires(T t) { ^ t.cpp:2:14: note: the required type ‘typename T::type’ would be ill-formed t.cpp:2:14: note: the required expression ‘t.i’ would be ill-formed t.cpp:2:14: note: the required expression ‘t.i2’ would be ill-formed t.cpp:2:14: note: the required expression ‘t.i3’ would be ill-formed t.cpp:2:14: note: the required expression ‘t.i4’ would be ill-formed t.cpp:2:14: note: the required expression ‘t.i5’ would be ill-formed t.cpp:14:6: note: ... 2 constraint errors not shown auto f(T t) { ^ This patch does not affect the diagnostics produced for failed static assertions of constraint satisfaction. Ideally, those diagnostics would be improved to list specific constraints as well.
