errorator: don't rely on non-evaluation of static_assert in constexpr branch

At least in clang, we appear to fail that static assert even with another
branch taken.  Instead, assert that one of the other conditions is true.

Signed-off-by: Samuel Just <sjust@redhat.com>

diff --git a/src/crimson/common/errorator.h b/src/crimson/common/errorator.h
index b7a7c8eff0031e480582119bb1d7c9894e0e9c89..15fffbb0a4165fe50f20d732ac99c26b16f465e3 100644 (file)
--- a/src/crimson/common/errorator.h
+++ b/src/crimson/common/errorator.h
@@ -299,7 +299,9 @@ static constexpr auto composer(FuncHead&& head, FuncTail&&... tail) {
                          std::forward<decltype(args)>(args)...);
     } else {
       static_assert(
-        _impl::always_false<FuncHead>::value, "composition is not exhaustive");
+       std::is_invocable_v<FuncHead, decltype(args)...> ||
+       (sizeof...(FuncTail) > 0),
+      "composition is not exhaustive");
     }
   };
 }