From 3fbada226294467581ed6ca0868c57bdfc2dfc7e Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Fri, 26 Jun 2026 15:58:49 +0100
Subject: Extract TypeCheckerSubtyping

---
 .../compiler/typechecker/TypeChecker.java          |  5 +---
 .../compiler/typechecker/TypeCheckerSubtyping.java | 12 +++++++++
 .../typechecker/TypeCheckerSubtypingTests.java     | 30 ++++++++++++++++++++++
 3 files changed, 43 insertions(+), 4 deletions(-)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
 create mode 100644 src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtypingTests.java

(limited to 'src')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
index 1b10de2..841c934 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -9,6 +9,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import static org.zwobble.hobgoblin.compiler.typechecker.NamespaceNameResolution.resolveNamespaceName;
+import static org.zwobble.hobgoblin.compiler.typechecker.TypeCheckerSubtyping.isSubtype;
 
 public class TypeChecker {
 
@@ -395,8 +396,4 @@ public class TypeChecker {
             );
         }
     }
-
-    private static boolean isSubtype(Type subtype, Type supertype) {
-        return subtype.equals(supertype);
-    }
 }
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
new file mode 100644
index 0000000..673d931
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtyping.java
@@ -0,0 +1,12 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.types.Type;
+
+class TypeCheckerSubtyping {
+    private TypeCheckerSubtyping() {
+    }
+
+    static boolean isSubtype(Type subtype, Type supertype) {
+        return subtype.equals(supertype);
+    }
+}
diff --git a/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtypingTests.java b/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtypingTests.java
new file mode 100644
index 0000000..35779fe
--- /dev/null
+++ b/src/test/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerSubtypingTests.java
@@ -0,0 +1,30 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.junit.jupiter.api.Test;
+import org.zwobble.hobgoblin.compiler.types.SimpleNativeType;
+
+import static org.zwobble.hobgoblin.compiler.typechecker.TypeCheckerSubtyping.isSubtype;
+import static org.zwobble.precisely.AssertThat.assertThat;
+import static org.zwobble.precisely.Matchers.equalTo;
+
+public class TypeCheckerSubtypingTests {
+    @Test
+    public void whenTypeIsEqualToTypeThenTypeIsSubtype() {
+        var nativeType1 = SimpleNativeType.builtin("Int32");
+        var nativeType2 = SimpleNativeType.builtin("Int32");
+
+        var isSubtype = isSubtype(nativeType1, nativeType2);
+
+        assertThat(isSubtype, equalTo(true));
+    }
+
+    @Test
+    public void distinctTypesAreNotSubtypes() {
+        var nativeType1 = SimpleNativeType.builtin("Int32");
+        var nativeType2 = SimpleNativeType.builtin("Int64");
+
+        var isSubtype = isSubtype(nativeType1, nativeType2);
+
+        assertThat(isSubtype, equalTo(false));
+    }
+}
-- 
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