1 files changed, 52 insertions, 0 deletions
diff --git a/src/test/java/org/shedlang/compiler/types/CommonSubtypeTests.java b/src/test/java/org/shedlang/compiler/types/CommonSubtypeTests.java
new file mode 100644
index 00000000..9095ca07
--- /dev/null
+++ b/src/test/java/org/shedlang/compiler/types/CommonSubtypeTests.java
@@ -0,0 +1,52 @@
+package org.shedlang.compiler.types;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.zwobble.precisely.AssertThat.assertThat;
+import static org.zwobble.precisely.Matchers.equalTo;
+
+public class CommonSubtypeTests {
+    @Test
+    public void commonSubtypeOfNoTypesIsTop() {
+        var result = Types.commonSubtype(List.of());
+
+        assertThat(result, equalTo(Types.TOP));
+    }
+
+    @Test
+    public void commonSubtypeOfSingleTypeIsThatType() {
+        var result = Types.commonSubtype(List.of(Types.INT));
+
+        assertThat(result, equalTo(Types.INT));
+    }
+
+    @Test
+    public void commonSubtypeOfRepeatedTypeIsThatType() {
+        var result = Types.commonSubtype(List.of(Types.INT, Types.INT, Types.INT));
+
+        assertThat(result, equalTo(Types.INT));
+    }
+
+    @Test
+    public void commonSubtypeOfUnrelatedTypesIsBottom() {
+        var result = Types.commonSubtype(List.of(Types.INT, Types.STRING));
+
+        assertThat(result, equalTo(Types.BOTTOM));
+    }
+
+    @Test
+    public void commonSubtypeOfTopAndOtherTypeIsOtherType() {
+        var result = Types.commonSubtype(List.of(Types.TOP, Types.INT));
+
+        assertThat(result, equalTo(Types.INT));
+    }
+
+    @Test
+    public void commonSubtypeOfOtherTypeAndTopIsOtherType() {
+        var result = Types.commonSubtype(List.of(Types.INT, Types.TOP));
+
+        assertThat(result, equalTo(Types.INT));
+    }
+}