https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79318
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Smaller testcase: struct A { int a; A () : a(0) {}; &operator int () { return a; }; }; int bar () { A a; return a; } clang++ indeed reports: pr79318.C:5:3: error: cannot specify any part of a return type in the declaration of a conversion function; put the complete type after 'operator' &operator int () { return a; }; ^ & [dcl.dcl]/12 indeed requires that conversion functions are declared with nodeclspec-function-declaration.