Reproduce miscompilation in #4685

crates/compiler/test_mono/generated/list_one_vs_one_spread_issue_4685.txt

@@ -0,0 +1,24 @@
+procedure Test.0 ():
+    let Test.8 : Str = "";
+    let Test.1 : List Str = Array [Test.8];
+    joinpoint Test.6:
+        let Test.4 : Str = "C";
+        ret Test.4;
+    in
+    joinpoint Test.5:
+        let Test.3 : Str = "B";
+        ret Test.3;
+    in
+    let Test.7 : U64 = lowlevel ListLen Test.1;
+    dec Test.1;
+    switch Test.7:
+        case 0:
+            let Test.2 : Str = "A";
+            ret Test.2;
+    
+        case 1:
+            jump Test.5;
+    
+        default:
+            jump Test.5;
+    

crates/compiler/test_mono/src/tests.rs

@@ -2120,3 +2120,17 @@ fn toplevel_accessor_fn_thunk() {
         "#
     )
 }
+
+#[mono_test]
+fn list_one_vs_one_spread_issue_4685() {
+    indoc!(
+        r#"
+        app "test" provides [main] to "./platform"
+
+        main = when [""] is
+            [] -> "A"
+            [_] -> "B"
+            [_, ..] -> "C"
+        "#
+    )
+}