From ce8dfcfecd0937fbf2f4d5b941517f3750f29af2 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sat, 30 Sep 2023 17:31:10 +0100
Subject: Add subtyping for bottom type

---
 .../compiler/typechecker/TypeCheckerSubTyping.java | 14 ++++++-
 .../typechecker/TypeCheckerIsSubTypeTests.java     | 48 ++++++++++++++++++++++
 2 files changed, 61 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
index c9576191..b73011ce 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerSubTyping.java
@@ -8,6 +8,18 @@ public class TypeCheckerSubTyping {
     }
 
     static boolean isSubType(Type subType, Type superType, TypeCheckerContext context) {
-        return superType == Types.TOP || subType.equals(superType);
+        if (superType == Types.TOP) {
+            return true;
+        }
+
+        if (subType == Types.BOTTOM) {
+            return true;
+        }
+
+        if (subType.equals(superType)) {
+            return true;
+        }
+
+        return false;
     }
 }
diff --git a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
index c80a861a..b7da683c 100644
--- a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
+++ b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
@@ -57,4 +57,52 @@ public class TypeCheckerIsSubTypeTests {
             equalTo(true)
         );
     }
+
+    @Test
+    public void topTypeIsSubTypeOfNoOtherTypes() {
+        var namespaceName = NamespaceName.of("Example", "Project");
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(Types.TOP, Types.INT, context), equalTo(false));
+        assertThat(
+            isSubType(
+                Types.TOP,
+                new StructType(namespaceName, "X"),
+                context
+            ),
+            equalTo(false)
+        );
+    }
+
+    @Test
+    public void bottomTypeIsSubTypeOfAllTypes() {
+        var namespaceName = NamespaceName.of("Example", "Project");
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(Types.BOTTOM, Types.INT, context), equalTo(true));
+        assertThat(
+            isSubType(
+                Types.BOTTOM,
+                new StructType(namespaceName, "X"),
+                context
+            ),
+            equalTo(true)
+        );
+    }
+
+    @Test
+    public void bottomTypeIsSuperTypeOfNoOtherTypes() {
+        var namespaceName = NamespaceName.of("Example", "Project");
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(Types.INT, Types.BOTTOM, context), equalTo(false));
+        assertThat(
+            isSubType(
+                new StructType(namespaceName, "X"),
+                Types.BOTTOM,
+                context
+            ),
+            equalTo(false)
+        );
+    }
 }
-- 
cgit v1.2.3