From 47e490407553acd51f7676fb90eaff0f440bc35b Mon Sep 17 00:00:00 2001
From: ayazhafiz <ayaz.hafiz.1@gmail.com>
Date: Sun, 6 Mar 2022 22:15:02 -0500
Subject: [PATCH] Chase aliases when checking for valid extension types

---
 compiler/can/src/annotation.rs     |  2 +-
 compiler/solve/tests/solve_expr.rs | 30 ++++++++++++++++++++++++++++++
 compiler/types/src/types.rs        |  7 ++++---
 reporting/tests/test_reporting.rs  |  4 ++--
 4 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/compiler/can/src/annotation.rs b/compiler/can/src/annotation.rs
index 1fbe8a3652..9ace3a1a7d 100644
--- a/compiler/can/src/annotation.rs
+++ b/compiler/can/src/annotation.rs
@@ -682,7 +682,7 @@ fn can_extension_type<'a>(
                 local_aliases,
                 references,
             );
-            if valid_extension_type(&ext_type) {
+            if valid_extension_type(&ext_type.shallow_dealias()) {
                 ext_type
             } else {
                 // Report an error but mark the extension variable to be inferred
diff --git a/compiler/solve/tests/solve_expr.rs b/compiler/solve/tests/solve_expr.rs
index 70b321afcd..4c4f36de14 100644
--- a/compiler/solve/tests/solve_expr.rs
+++ b/compiler/solve/tests/solve_expr.rs
@@ -5543,4 +5543,34 @@ mod solve_expr {
             "Str",
         )
     }
+
+    #[test]
+    fn record_extension_variable_is_alias() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                Other a b : { y: a, z: b }
+
+                f : { x : Str }(Other Str Str)
+                f
+                "#
+            ),
+            r#"{ x : Str, y : Str, z : Str }"#,
+        )
+    }
+
+    #[test]
+    fn tag_extension_variable_is_alias() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                Other : [ B, C ]
+
+                f : [ A ]Other
+                f
+                "#
+            ),
+            r#"[ A, B, C ]"#,
+        )
+    }
 }
diff --git a/compiler/types/src/types.rs b/compiler/types/src/types.rs
index 6bc9b7ed9e..c6f4f360f3 100644
--- a/compiler/types/src/types.rs
+++ b/compiler/types/src/types.rs
@@ -722,10 +722,11 @@ impl Type {
 
     /// a shallow dealias, continue until the first constructor is not an alias.
     pub fn shallow_dealias(&self) -> &Self {
-        match self {
-            Type::Alias { actual, .. } => actual.shallow_dealias(),
-            _ => self,
+        let mut result = self;
+        while let Type::Alias { actual, .. } = result {
+            result = actual;
         }
+        result
     }
 
     pub fn instantiate_aliases(
diff --git a/reporting/tests/test_reporting.rs b/reporting/tests/test_reporting.rs
index 4f1fc68969..247db7a0ec 100644
--- a/reporting/tests/test_reporting.rs
+++ b/reporting/tests/test_reporting.rs
@@ -8546,7 +8546,7 @@ I need all branches in an `if` to have the same type!
         report_problem_as(
             indoc!(
                 r#"
-                f : [ A ]U32
+                f : [ A ]Str
                 f
                 "#
             ),
@@ -8556,7 +8556,7 @@ I need all branches in an `if` to have the same type!
 
                 This tag union extension type is invalid:
 
-                1│  f : [ A ]U32
+                1│  f : [ A ]Str
                              ^^^
 
                 Note: A tag union extension variable can only contain a type variable