Re: [PATCH][RFC] Instrument function exit with __builtin_unreachable in C++.

[Jason Merrill]

On 10/18/2017 09:07 AM, Martin Liška wrote:
@@ -1182,7 +1182,13 @@ cxx_eval_builtin_function_call (const constexpr_ctx 
*ctx, tree t, tree fun,
        {
          new_call = build_call_array_loc (EXPR_LOCATION (t), TREE_TYPE (t),
                                           CALL_EXPR_FN (t), nargs, args);
-         error ("%q+E is not a constant expression", new_call);
+
+         /* Do not allow__builtin_unreachable in constexpr function.  */
+         if (DECL_FUNCTION_CODE (fun) == BUILT_IN_UNREACHABLE
+             && EXPR_LOCATION (t) == BUILTINS_LOCATION)
+           error ("constexpr call flows off the end of the function");
+         else
+           error ("%q+E is not a constant expression", new_call);

You don't need to build new_call in the new case, since you don't use it.

Also, please adjust the comment to say that a __builtin_unreachable call 
with BUILTINS_LOCATION comes from cp_maybe_instrument_return.

OK with those changes.

Jason