fix alias instantiation problem

compiler/can/src/def.rs

@@ -1717,7 +1717,15 @@ fn correct_mutual_recursive_type_alias<'a>(
     let capacity = original_aliases.len();
     let mut matrix = ReferenceMatrix::new(capacity);
 
-    let (symbols_introduced, mut aliases) = original_aliases.unzip();
+    // It is important that we instantiate with the aliases as they occur in the source.
+    // We must not instantiate A into B, then use the updated B to instantiate into C
+    //
+    // That is because B could be used different places. We don't want one place
+    // to mess with another.
+    let (symbols_introduced, uninstantiated_aliases) = original_aliases.unzip();
+
+    // the aliases that we will instantiate into
+    let mut aliases = uninstantiated_aliases.clone();
 
     for (index, alias) in aliases.iter().enumerate() {
         for referenced in alias.typ.symbols() {
@@ -1765,7 +1773,9 @@ fn correct_mutual_recursive_type_alias<'a>(
             std::mem::swap(&mut alias_type, &mut aliases[index].typ);
 
             let can_instantiate_symbol = |s| match symbols_introduced.iter().position(|i| *i == s) {
-                Some(s_index) if to_instantiate_bitvec[s_index] => aliases.get(s_index),
+                Some(s_index) if to_instantiate_bitvec[s_index] => {
+                    uninstantiated_aliases.get(s_index)
+                }
                 _ => None,
             };
 

compiler/solve/tests/solve_expr.rs

@@ -5297,7 +5297,24 @@ mod solve_expr {
 
     #[test]
     fn issue_2458() {
-        // TODO: the order of the alias definitions matters
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                Foo a : [ Blah (Result (Bar a) { val: a }) ]
+                Bar a : Foo a
+
+                v : Bar U8
+                v = Blah (Ok (Blah (Err { val: 1 })))
+
+                v
+                "#
+            ),
+            "Bar U8",
+        )
+    }
+
+    #[test]
+    fn issue_2458_swapped_order() {
         infer_eq_without_problem(
             indoc!(
                 r#"