diff --git a/compiler/can/src/def.rs b/compiler/can/src/def.rs
index bcdde10c17..04d5a2ccaa 100644
--- a/compiler/can/src/def.rs
+++ b/compiler/can/src/def.rs
@@ -1966,13 +1966,12 @@ fn correct_mutual_recursive_type_alias<'a>(
             // Don't try to instantiate the alias itself in its definition.
             let original_alias_def = to_instantiate.remove(&rec).unwrap();
 
+            let helper = |s| to_instantiate.get(&s);
+
             let mut new_lambda_sets = ImSet::default();
-            alias.typ.instantiate_aliases(
-                alias.region,
-                &to_instantiate,
-                var_store,
-                &mut new_lambda_sets,
-            );
+            alias
+                .typ
+                .instantiate_aliases(alias.region, &helper, var_store, &mut new_lambda_sets);
 
             for lambda_set_var in new_lambda_sets {
                 alias
diff --git a/compiler/types/src/types.rs b/compiler/types/src/types.rs
index 0f3536cd54..0e51c2dda4 100644
--- a/compiler/types/src/types.rs
+++ b/compiler/types/src/types.rs
@@ -120,13 +120,15 @@ impl RecordField<Type> {
         }
     }
 
-    pub fn instantiate_aliases(
+    pub fn instantiate_aliases<'a, F>(
         &mut self,
         region: Region,
-        aliases: &ImMap<Symbol, Alias>,
+        aliases: &'a F,
         var_store: &mut VarStore,
         introduced: &mut ImSet<Variable>,
-    ) {
+    ) where
+        F: Fn(Symbol) -> Option<&'a Alias>,
+    {
         use RecordField::*;
 
         match self {
@@ -168,13 +170,15 @@ impl LambdaSet {
         &mut self.0
     }
 
-    fn instantiate_aliases(
+    fn instantiate_aliases<'a, F>(
         &mut self,
         region: Region,
-        aliases: &ImMap<Symbol, Alias>,
+        aliases: &'a F,
         var_store: &mut VarStore,
         introduced: &mut ImSet<Variable>,
-    ) {
+    ) where
+        F: Fn(Symbol) -> Option<&'a Alias>,
+    {
         self.0
             .instantiate_aliases(region, aliases, var_store, introduced)
     }
@@ -1064,13 +1068,15 @@ impl Type {
         result
     }
 
-    pub fn instantiate_aliases(
+    pub fn instantiate_aliases<'a, F>(
         &mut self,
         region: Region,
-        aliases: &ImMap<Symbol, Alias>,
+        aliases: &'a F,
         var_store: &mut VarStore,
         new_lambda_set_variables: &mut ImSet<Variable>,
-    ) {
+    ) where
+        F: Fn(Symbol) -> Option<&'a Alias>,
+    {
         use Type::*;
 
         match self {
@@ -1138,7 +1144,7 @@ impl Type {
                 );
             }
             Apply(symbol, args, _) => {
-                if let Some(alias) = aliases.get(symbol) {
+                if let Some(alias) = aliases(*symbol) {
                     // TODO switch to this, but we still need to check for recursion with the
                     // `else` branch
                     if false {