The recursive call to f() below, from inside g(), is not recognized as either happening in tail position or as causing an error, hence producing code consuming an non-constant amount of stack space:

val foo = false; @tailrec def f(): Int = { if (foo) f() else g() }; def g(): Int = f();

@tailrec def f[T](l: List[T]): Int = { val v = l match { case Nil => 0; case a :: l1 => g(l1) }; f[T](Nil)}; def g[T](l1: List[T]): Int = f(l1);

If you wonder why would one care, the original example is in unificationHelper here — I noticed because of a StackOverflowException:

https://github.com/inc-lc/ilc-scala/blob/597a1e6b53dedf9d626061d7b7a9e9da66df499f/src/main/scala/ilc/feature/inference/Inference.scala#L126

In all cases, the code can be made tail-recursive by inlining. However, I imagine this cannot be supported easily. In that case, the code should simply be rejected by the analysis done for tailrec.