Expose type mismatches between recursive types and types that aren't

Closes #2166

compiler/unify/src/unify.rs

@@ -285,7 +285,12 @@ fn unify_structure(
                 // unify the structure with this unrecursive tag union
                 unify_pool(subs, pool, ctx.first, *structure, ctx.mode)
             }
-            _ => todo!("rec structure {:?}", &flat_type),
+            // Only tag unions can be recursive; everything else is an error.
+            _ => mismatch!(
+                "trying to unify {:?} with recursive type var {:?}",
+                &flat_type,
+                structure
+            ),
         },
 
         Structure(ref other_flat_type) => {

reporting/tests/test_reporting.rs

@@ -7201,4 +7201,41 @@ I need all branches in an `if` to have the same type!
             ),
         )
     }
+
+    #[test]
+    fn unify_recursive_with_nonrecursive() {
+        report_problem_as(
+            indoc!(
+                r#"
+                Job : [ @Job { inputs : List Job } ]
+
+                job : { inputs : List Str } -> Job
+                job = \{ inputs } ->
+                    @Job { inputs }
+
+                job { inputs: [ "build", "test" ] }
+                "#
+            ),
+            indoc!(
+                r#"
+                ── TYPE MISMATCH ───────────────────────────────────────────────────────────────
+
+                Something is off with the body of the `job` definition:
+
+                3│  job : { inputs : List Str } -> Job
+                4│  job = \{ inputs } ->
+                5│      @Job { inputs }
+                        ^^^^^^^^^^^^^^^
+
+                This `@Job` private tag application has the type:
+
+                    [ @Job { inputs : List Str } ]
+
+                But the type annotation on `job` says it should be:
+
+                    [ @Job { inputs : List a } ] as a
+                "#
+            ),
+        )
+    }
 }