Correct when fixpoint-fixed type variables can be reunified

With fixpoint-fixing, we don't want to re-unify type variables that were
just fixed, because doing so may change their shapes in ways that we
explicitly just set them up not to be changed (as fixpoint-fixing
clobbers type variable contents).

However, this restriction need only apply when we re-unify two type
variables that were both involved in the same fixpoint-fixing cycle. If
we have a type variable T that was involved in fixpoint-fixing, and we
unify it with U that wasn't, we know that the $U \notin \bar{T}$, where
$\bar{T}$ is the recursive closure of T. In these cases, we do want to
permit the usual in-band unification of $T \sim U$.

crates/compiler/solve/tests/solve_expr.rs

@@ -8283,4 +8283,41 @@ mod solve_expr {
             "MDict v -> MDict v | v has Eq",
         );
     }
+
+    #[test]
+    fn unify_types_with_fixed_fixpoints_outside_fixing_region() {
+        infer_queries!(indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            Input := [
+                FromJob Job
+            ]
+
+            Job := [
+                Job (List Input)
+            ]
+
+            job : List Input -> Job
+            job = \inputs ->
+                @Job (Job inputs)
+
+            helloWorld : Job
+            helloWorld =
+                @Job ( Job [ @Input (FromJob greeting) ] )
+                #            ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+            greeting : Job
+            greeting =
+                job []
+
+            main = (\_ -> "Which platform am I running on now?\n") helloWorld
+            "#
+        ),
+        @r###"
+        @Input (FromJob greeting) : [FromJob ([Job (List [FromJob a])] as a)]
+        "###
+        print_only_under_alias: true
+        )
+    }
 }

crates/compiler/test_gen/src/gen_tags.rs

@@ -2047,3 +2047,39 @@ fn issue_4077_fixed_fixpoint() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn unify_types_with_fixed_fixpoints_outside_fixing_region() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            app "test" provides [main] to "./platform"
+
+            Input := [
+                FromJob Job (List Str),
+            ]
+
+            Job := [
+                Job (List Input)
+            ]
+
+            job : List Input -> Job
+            job = \inputs ->
+                @Job (Job inputs)
+
+            helloWorld : Job
+            helloWorld =
+                @Job ( Job [ @Input (FromJob greeting []) ] )
+
+            greeting : Job
+            greeting =
+                job []
+
+            main = (\_ -> "OKAY") helloWorld
+            "#
+        ),
+        RocStr::from("OKAY"),
+        RocStr
+    );
+}

crates/compiler/unify/src/unify.rs

@@ -2814,8 +2814,13 @@ fn unify_shared_tags_merge_new<M: MetaCollector>(
     new_ext_var: Variable,
     recursion_var: Rec,
 ) -> Outcome<M> {
-    let was_fixed = env.was_fixed(ctx.first) || env.was_fixed(ctx.second);
-    if was_fixed {
+    if env.was_fixed(ctx.first) && env.was_fixed(ctx.second) {
+        // Both of the tags we're looking at were just involved in fixpoint-fixing, so their types
+        // should be aligned. As such, do not attempt to unify them and update the recursion
+        // pointer again.
+        debug_assert!(env
+            .subs
+            .equivalent_without_compacting(ctx.first, ctx.second));
         return Default::default();
     }