Reproduce #3560

Part of #3560

crates/compiler/mono/src/decision_tree.rs

@@ -1343,7 +1343,7 @@ fn test_to_equality<'a>(
                         }),
                     )
                 }
-                _ => unreachable!("{:?}", (cond_layout, union)),
+                _ => unreachable!("{:#?}", (cond_layout, union, test_layout, path)),
             }
         }
 

crates/compiler/solve/tests/solve_expr.rs

@@ -7357,4 +7357,24 @@ mod solve_expr {
             "List (A U8)",
         );
     }
+
+    #[test]
+    fn issue_3560_nested_tag_union() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test"
+                    imports [Encode.{ toEncoder }, Json]
+                    provides [main] to "./platform"
+
+                main =
+                    when Wrapper (Payload "err") is
+                    #    ^^^^^^^^^^^^^^^^^^^^^^^
+                        Wrapper (Payload str) -> str
+                        Wrapper (AlternatePayload str) -> str
+                "#
+            ),
+            &[r#"Wrapper (Payload "err") : [Wrapper [AlternatePayload Str, Payload Str]]"#,]
+        )
+    }
 }

crates/compiler/test_mono/src/tests.rs

@@ -1884,3 +1884,19 @@ fn encode_derived_tag_two_payloads_string() {
         "#
     )
 }
+
+#[mono_test]
+fn issue_3560_nested_tag_union() {
+    indoc!(
+        r#"
+        app "test"
+            imports [Encode.{ toEncoder }, Json]
+            provides [main] to "./platform"
+
+        main =
+            when Wrapper (Payload "err") is
+                Wrapper (Payload str) -> str
+                Wrapper (AlternatePayload str) -> str
+        "#
+    )
+}