feat: use bounds of type system where possible (#899)

### Summary of Changes

* `isSubtypeOf(Nothing, X)` now always returns `true`.
* `isSubtypeOf(X, Any?)` now always returns `true`.
* `lowestCommonSupertype` now returns `Any` or `Any?` if given
`$unknown` or some other unexpected type, instead of propagating
`$unknown`.

packages/safe-ds-lang/src/language/typing/safe-ds-type-checker.ts

@@ -61,9 +61,13 @@ export class SafeDsTypeChecker {
      * Checks whether {@link type} is a subtype of {@link other}.
      */
     isSubtypeOf = (type: Type, other: Type, options: TypeCheckOptions = {}): boolean => {
-        if (type === UnknownType || other === UnknownType) {
+        if (type.equals(this.coreTypes.Nothing) || other.equals(this.coreTypes.AnyOrNull)) {
+            return true;
+        } else if (type === UnknownType || other === UnknownType) {
             return false;
-        } else if (other instanceof TypeParameterType) {
+        }
+
+        if (other instanceof TypeParameterType) {
             const otherUpperBound = this.typeComputer().computeUpperBound(other);
             return this.isSubtypeOf(type, otherUpperBound, options);
         } else if (other instanceof UnionType) {
@@ -262,7 +266,9 @@ export class SafeDsTypeChecker {
         other: Type,
         options: TypeCheckOptions,
     ): boolean {
-        if (other instanceof NamedTupleType) {
+        if (other instanceof ClassType) {
+            return other.declaration === this.builtinClasses.Any;
+        } else if (other instanceof NamedTupleType) {
             return (
                 type.length === other.length &&
                 type.entries.every((typeEntry, i) => {
@@ -281,6 +287,8 @@ export class SafeDsTypeChecker {
     private staticTypeIsSubtypeOf(type: StaticType, other: Type, options: TypeCheckOptions): boolean {
         if (other instanceof CallableType) {
             return this.isSubtypeOf(this.associatedCallableTypeForStaticType(type), other, options);
+        } else if (other instanceof ClassType) {
+            return other.declaration === this.builtinClasses.Any;
         } else {
             return type.equals(other);
         }

packages/safe-ds-lang/src/language/typing/safe-ds-type-computer.ts

@@ -742,14 +742,22 @@ export class SafeDsTypeComputer {
             }
         });
 
-        // Includes type with unknown supertype
+        // Group types by their kind
         const groupedTypes = this.groupTypes(replacedTypes);
-        if (groupedTypes.hasTypeWithUnknownSupertype) {
-            return UnknownType;
+
+        // Includes unknown type
+        if (groupedTypes.containsUnknownType) {
+            return this.coreTypes.AnyOrNull;
         }
 
+        // The result must be nullable if any of the types is nullable
         const isNullable = replacedTypes.some((it) => it.isExplicitlyNullable);
 
+        // Includes unhandled type
+        if (groupedTypes.containsUnhandledType) {
+            return this.Any(isNullable);
+        }
+
         // Class-based types
         if (!isEmpty(groupedTypes.classTypes)) {
             if (!isEmpty(groupedTypes.enumTypes) || !isEmpty(groupedTypes.enumVariantTypes)) {
@@ -786,7 +794,8 @@ export class SafeDsTypeComputer {
             classTypes: [],
             enumTypes: [],
             enumVariantTypes: [],
-            hasTypeWithUnknownSupertype: false,
+            containsUnhandledType: false,
+            containsUnknownType: false,
         };
 
         for (const type of types) {
@@ -803,11 +812,14 @@ export class SafeDsTypeComputer {
                 if (classType instanceof ClassType) {
                     result.classTypes.push(classType);
                 } else {
-                    result.hasTypeWithUnknownSupertype = true;
+                    /* c8 ignore next 2 */
+                    result.containsUnknownType = true;
                 }
+            } else if (type === UnknownType) {
+                result.containsUnknownType = true;
             } else {
-                // Other types don't have a clear lowest common supertype
-                result.hasTypeWithUnknownSupertype = true;
+                // Since these types don't occur in legal programs, we don't need to handle them better
+                result.containsUnhandledType = true;
                 return result;
             }
         }
@@ -1022,7 +1034,7 @@ export class SafeDsTypeComputer {
             current = this.parentClassType(current);
         }
 
-        const Any = this.coreTypes.Any.withExplicitNullability(type.isExplicitlyNullable);
+        const Any = this.Any(type.isExplicitlyNullable);
         if (Any instanceof ClassType && !visited.has(Any.declaration)) {
             yield Any;
         }
@@ -1072,7 +1084,8 @@ interface GroupTypesResult {
     classTypes: ClassType[];
     enumTypes: EnumType[];
     enumVariantTypes: EnumVariantType[];
-    hasTypeWithUnknownSupertype: boolean;
+    containsUnhandledType: boolean;
+    containsUnknownType: boolean;
 }
 
 const NO_SUBSTITUTIONS: TypeParameterSubstitutions = new Map();

packages/safe-ds-lang/tests/language/typing/type checker/isSubOrSupertypeOf.test.ts

@@ -103,6 +103,7 @@ const basic = async (): Promise<IsSubOrSupertypeOfTest[]> => {
     const enumVariantType2 = typeComputer.computeType(enumVariant2) as EnumVariantType;
 
     return [
+        // Callable type to callable type
         {
             type1: callableType1,
             type2: callableType1,
@@ -526,6 +527,22 @@ const basic = async (): Promise<IsSubOrSupertypeOfTest[]> => {
             type2: enumType1,
             expected: false,
         },
+        // Named tuple type to class type
+        {
+            type1: factory.createNamedTupleType(),
+            type2: classType1,
+            expected: false,
+        },
+        {
+            type1: factory.createNamedTupleType(),
+            type2: coreTypes.Any,
+            expected: true,
+        },
+        {
+            type1: factory.createNamedTupleType(),
+            type2: coreTypes.AnyOrNull,
+            expected: true,
+        },
         // Named tuple type to named tuple type
         {
             type1: factory.createNamedTupleType(),
@@ -604,6 +621,22 @@ const basic = async (): Promise<IsSubOrSupertypeOfTest[]> => {
             type2: callableType12,
             expected: true,
         },
+        // Static type to class type
+        {
+            type1: factory.createStaticType(classType1),
+            type2: classType1,
+            expected: false,
+        },
+        {
+            type1: factory.createStaticType(classType1),
+            type2: coreTypes.Any,
+            expected: true,
+        },
+        {
+            type1: factory.createStaticType(classType1),
+            type2: coreTypes.AnyOrNull,
+            expected: true,
+        },
         // Static type to static type
         {
             type1: factory.createStaticType(classType1),
@@ -658,6 +691,16 @@ const basic = async (): Promise<IsSubOrSupertypeOfTest[]> => {
             type2: UnknownType,
             expected: false,
         },
+        {
+            type1: coreTypes.Nothing,
+            type2: UnknownType,
+            expected: true,
+        },
+        {
+            type1: UnknownType,
+            type2: coreTypes.AnyOrNull,
+            expected: true,
+        },
     ];
 };
 
@@ -1011,7 +1054,7 @@ const typeParameterTypes = async (): Promise<IsSubOrSupertypeOfTest[]> => {
         {
             type1: unresolved,
             type2: unbounded,
-            expected: false,
+            expected: true,
         },
         {
             type1: coreTypes.AnyOrNull,
@@ -1107,7 +1150,7 @@ const typeParameterTypes = async (): Promise<IsSubOrSupertypeOfTest[]> => {
         {
             type1: coreTypes.Nothing,
             type2: unresolved,
-            expected: false,
+            expected: true,
         },
 
         // Compare to some other type

packages/safe-ds-lang/tests/resources/typing/lowest common supertype/unhandled type/main.sdstest

@@ -0,0 +1,24 @@
+package tests.typing.lowestCommonSupertype.unhandledType
+
+class C
+@Pure fun f() -> (r1: Int, r2: Int)
+
+segment mySegment() {
+    // $TEST$ serialization List<Any>
+    »[1, f]«;
+
+    // $TEST$ serialization List<Any?>
+    »[null, f]«;
+
+    // $TEST$ serialization List<Any>
+    »[1, C]«;
+
+    // $TEST$ serialization List<Any?>
+    »[null, C]«;
+
+    // $TEST$ serialization List<Any>
+    »[1, f()]«;
+
+    // $TEST$ serialization List<Any?>
+    »[null, f()]«;
+}

packages/safe-ds-lang/tests/resources/typing/lowest common supertype/unknown type/main.sdstest

@@ -0,0 +1,9 @@
+package tests.typing.lowestCommonSupertype.unknownType
+
+segment mySegment() {
+    // $TEST$ serialization List<Any?>
+    »[1, unknown]«;
+
+    // $TEST$ serialization List<Any?>
+    »[null, unknown]«;
+}