add test for rust-lang#122301 to cover behavior that's on stable

if this ought to be broken it should at least happen intentionally

tests/ui/consts/control-flow/dead_branches_dont_eval.rs

@@ -0,0 +1,26 @@
+//@ check-pass
+
+// issue 122301 - currently the only way to supress
+// const eval and codegen of code conditional on some other const
+
+struct Foo<T, const N: usize>(T);
+
+impl<T, const N: usize> Foo<T, N> {
+    const BAR: () = if N == 0 {
+        panic!()
+    };
+}
+
+struct Invoke<T, const N: usize>(T);
+
+impl<T, const N: usize> Invoke<T, N> {
+    const FUN: fn() = if N != 0 {
+        || Foo::<T, N>::BAR
+    } else {
+        || {}
+    };
+}
+
+fn main() {
+    Invoke::<(), 0>::FUN();
+}