https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71543
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> --- GCC 10 and above reject the code: <source>:4:28: error: placeholder type not allowed in this context 4 | concept bool C2 = requires(C1 c1) {}; | ^~ <source>:4:36: error: expected primary-expression before '}' token 4 | concept bool C2 = requires(C1 c1) {}; | ^ <source>:4:36: error: expected ';' before '}' token 4 | concept bool C2 = requires(C1 c1) {}; | ^ | ;