https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67026
--- Comment #3 from Anders Granlund <anders.granlund.0 at gmail dot com> --- (In reply to Andrew Pinski from comment #2) > Actually wait. I think this is invalid and clang is incorrect in not > rejecting it. Because you have a call to a non constexpr in a constexpr > function; does not matter if it is after a return or not. My program is valid. Just having a call expression with a non-constexpr function inside the body of a constexpr function is not in it self a reason for the program to be ill-formed. The c++ standard is quite permissive about what a function body of a constexpr function can contain, see [dcl.constexpr]p3 (http://eel.is/c++draft/dcl.constexpr#3). The program would however be ill-formed with no diagnostics required, if the constexpr function could never be called without calling the non-constexpr function. For details, see [dcl.constexpr]p5 (http://eel.is/c++draft/dcl.constexpr#5). Also the program wold be ill-formed, if the constexpr function needs to be called when evaluating an expression that needs to be a constant expression, and that call would result in a call to the non-constexpr function. For details, see [expr.const]p2 (http://eel.is/c++draft/expr.const#2) (item 2 in the list). I choose the return type void to avoid having to return a value in f. The test case works with int as return type also. void g() {} constexpr int f() { return 0; g(); } int main() {} Anyways GCC supports the return type void for constexpr functions. Also relaxed requirements on constexpr functions have been implemented since version 5 of GCC according to this: https://gcc.gnu.org/projects/cxx1y.html
