Add procedure for checking type equivalence

src/ast/TypeSystem.cpp

@@ -367,4 +367,8 @@ std::optional<TypeAttribute> getTypeAttribute(const TypeSet& type) {
     return {};
 }
 
+bool areEquivalentTypes(const Type& a, const Type& b) {
+    return isSubtypeOf(a, b) && isSubtypeOf(b, a);
+}
+
 }  // end of namespace souffle

src/ast/TypeSystem.h

@@ -511,4 +511,10 @@ TypeSet getGreatestCommonSubtypes(const Types&... types) {
  */
 bool haveCommonSupertype(const Type& a, const Type& b);
 
+/**
+ * Determine if two types are equivalent.
+ * That is, check if a <: b and b <: a
+ */
+bool areEquivalentTypes(const Type& a, const Type& b);
+
 }  // end namespace souffle

src/tests/type_system_test.cpp

@@ -255,4 +255,13 @@ TEST(TypeSystem, RecordSubsets) {
     EXPECT_EQ("{A}", toString(getGreatestCommonSubtypes(A, R)));
 }
 
+TEST(TypeSystem, EquivTypes) {
+    TypeEnvironment env;
+
+    auto& A = env.createType<SubsetType>("A", env.getType("number"));
+    auto& U = env.createType<UnionType>("U", toVector(dynamic_cast<const Type*>(&A)));
+
+    EXPECT_TRUE(areEquivalentTypes(A, U));
+}
+
 }  // namespace souffle::test