feat: various checks related to inheritance (#633)

Closes partially #543 

### Summary of Changes

Show an error if a class
* inherits from multiple types,
* inherits from something that is not a class, or
* inherits from itself.

---------

Co-authored-by: megalinter-bot <129584137+megalinter-bot@users.noreply.github.com>

Author: lars-reimann

package.json

@@ -69,7 +69,12 @@
                 "scopeName": "source.safe-ds",
                 "path": "./syntaxes/safe-ds.tmLanguage.json"
             }
-        ]
+        ],
+        "configurationDefaults": {
+            "[safe-ds]": {
+                "editor.wordSeparators": "`~!@#$%^&*()-=+[]{}\\|;:'\",.<>/?»«"
+            }
+        }
     },
     "type": "module",
     "main": "out/extension/main.cjs",

src/language/helpers/nodeProperties.ts

@@ -43,6 +43,7 @@ import {
     SdsResult,
     SdsResultList,
     SdsStatement,
+    SdsType,
     SdsTypeArgument,
     SdsTypeArgumentList,
     SdsTypeParameter,
@@ -165,6 +166,10 @@ export const parametersOrEmpty = (node: SdsCallable | undefined): SdsParameter[]
     return node?.parameterList?.parameters ?? [];
 };
 
+export const parentTypesOrEmpty = (node: SdsClass | undefined): SdsType[] => {
+    return node?.parentTypeList?.parentTypes ?? [];
+};
+
 export const placeholdersOrEmpty = (node: SdsBlock | undefined): SdsPlaceholder[] => {
     return stream(statementsOrEmpty(node))
         .filter(isSdsAssignment)

src/language/safe-ds-module.ts

@@ -21,6 +21,7 @@ import { SafeDsClasses } from './builtins/safe-ds-classes.js';
 import { SafeDsPackageManager } from './workspace/safe-ds-package-manager.js';
 import { SafeDsNodeMapper } from './helpers/safe-ds-node-mapper.js';
 import { SafeDsAnnotations } from './builtins/safe-ds-annotations.js';
+import { SafeDsClassHierarchy } from './typing/safe-ds-class-hierarchy.js';
 
 /**
  * Declaration of custom services - add your own service classes here.
@@ -34,6 +35,7 @@ export type SafeDsAddedServices = {
         NodeMapper: SafeDsNodeMapper;
     };
     types: {
+        ClassHierarchy: SafeDsClassHierarchy;
         TypeComputer: SafeDsTypeComputer;
     };
     workspace: {
@@ -71,6 +73,7 @@ export const SafeDsModule: Module<SafeDsServices, PartialLangiumServices & SafeD
         ScopeProvider: (services) => new SafeDsScopeProvider(services),
     },
     types: {
+        ClassHierarchy: (services) => new SafeDsClassHierarchy(services),
         TypeComputer: (services) => new SafeDsTypeComputer(services),
     },
     workspace: {

src/language/typing/safe-ds-class-hierarchy.ts

@@ -0,0 +1,60 @@
+import { SafeDsServices } from '../safe-ds-module.js';
+import { SafeDsClasses } from '../builtins/safe-ds-classes.js';
+import { SdsClass } from '../generated/ast.js';
+import { stream, Stream } from 'langium';
+import { parentTypesOrEmpty } from '../helpers/nodeProperties.js';
+import { SafeDsTypeComputer } from './safe-ds-type-computer.js';
+import { ClassType } from './model.js';
+
+export class SafeDsClassHierarchy {
+    private readonly builtinClasses: SafeDsClasses;
+    private readonly typeComputer: SafeDsTypeComputer;
+
+    constructor(services: SafeDsServices) {
+        this.builtinClasses = services.builtins.Classes;
+        this.typeComputer = services.types.TypeComputer;
+    }
+
+    /**
+     * Returns a stream of all superclasses of the given class. The class itself is not included in the stream unless
+     * there is a cycle in the inheritance hierarchy. Direct ancestors are returned first, followed by their ancestors
+     * and so on.
+     */
+    streamSuperclasses(node: SdsClass | undefined): Stream<SdsClass> {
+        if (!node) {
+            return stream();
+        }
+
+        const capturedThis = this;
+        const generator = function* () {
+            const visited = new Set<SdsClass>();
+            let current = capturedThis.parentClassOrUndefined(node);
+            while (current && !visited.has(current)) {
+                yield current;
+                visited.add(current);
+                current = capturedThis.parentClassOrUndefined(current);
+            }
+
+            const anyClass = capturedThis.builtinClasses.Any;
+            if (anyClass && node !== anyClass && !visited.has(anyClass)) {
+                yield anyClass;
+            }
+        };
+
+        return stream(generator());
+    }
+
+    /**
+     * Returns the parent class of the given class, or undefined if there is no parent class. Only the first parent
+     * type is considered, i.e. multiple inheritance is not supported.
+     */
+    private parentClassOrUndefined(node: SdsClass | undefined): SdsClass | undefined {
+        const [firstParentType] = parentTypesOrEmpty(node);
+        const computedType = this.typeComputer.computeType(firstParentType);
+        if (computedType instanceof ClassType) {
+            return computedType.sdsClass;
+        }
+
+        return undefined;
+    }
+}

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

@@ -85,19 +85,22 @@ import {
 
 export class SafeDsTypeComputer {
     private readonly astNodeLocator: AstNodeLocator;
-    private readonly coreClasses: SafeDsClasses;
+    private readonly builtinClasses: SafeDsClasses;
     private readonly nodeMapper: SafeDsNodeMapper;
 
     private readonly typeCache: WorkspaceCache<string, Type>;
 
     constructor(services: SafeDsServices) {
         this.astNodeLocator = services.workspace.AstNodeLocator;
-        this.coreClasses = services.builtins.Classes;
+        this.builtinClasses = services.builtins.Classes;
         this.nodeMapper = services.helpers.NodeMapper;
 
         this.typeCache = new WorkspaceCache(services.shared);
     }
 
+    /**
+     * Computes the type of the given node.
+     */
     computeType(node: AstNode | undefined): Type {
         if (!node) {
             return UnknownType;
@@ -507,35 +510,35 @@ export class SafeDsTypeComputer {
     // -----------------------------------------------------------------------------------------------------------------
 
     private AnyOrNull(): Type {
-        return this.createCoreType(this.coreClasses.Any, true);
+        return this.createCoreType(this.builtinClasses.Any, true);
     }
 
     private Boolean(): Type {
-        return this.createCoreType(this.coreClasses.Boolean);
+        return this.createCoreType(this.builtinClasses.Boolean);
     }
 
     private Float(): Type {
-        return this.createCoreType(this.coreClasses.Float);
+        return this.createCoreType(this.builtinClasses.Float);
     }
 
     private Int(): Type {
-        return this.createCoreType(this.coreClasses.Int);
+        return this.createCoreType(this.builtinClasses.Int);
     }
 
     private List(): Type {
-        return this.createCoreType(this.coreClasses.List);
+        return this.createCoreType(this.builtinClasses.List);
     }
 
     private Map(): Type {
-        return this.createCoreType(this.coreClasses.Map);
+        return this.createCoreType(this.builtinClasses.Map);
     }
 
     private NothingOrNull(): Type {
-        return this.createCoreType(this.coreClasses.Nothing, true);
+        return this.createCoreType(this.builtinClasses.Nothing, true);
     }
 
     private String(): Type {
-        return this.createCoreType(this.coreClasses.String);
+        return this.createCoreType(this.builtinClasses.String);
     }
 
     private createCoreType(coreClass: SdsClass | undefined, isNullable: boolean = false): Type {

src/language/validation/experimentalLanguageFeature.ts src/language/validation/experimentalLanguageFeatures.ts

@@ -0,0 +1,55 @@
+import { ValidationAcceptor } from 'langium';
+import { SdsClass } from '../generated/ast.js';
+import { parentTypesOrEmpty } from '../helpers/nodeProperties.js';
+import { isEmpty } from 'radash';
+import { SafeDsServices } from '../safe-ds-module.js';
+import { ClassType, UnknownType } from '../typing/model.js';
+
+export const CODE_INHERITANCE_CYCLE = 'inheritance/cycle';
+export const CODE_INHERITANCE_MULTIPLE_INHERITANCE = 'inheritance/multiple-inheritance';
+export const CODE_INHERITANCE_NOT_A_CLASS = 'inheritance/not-a-class';
+
+export const classMustOnlyInheritASingleClass = (services: SafeDsServices) => {
+    const typeComputer = services.types.TypeComputer;
+    const computeType = typeComputer.computeType.bind(typeComputer);
+
+    return (node: SdsClass, accept: ValidationAcceptor): void => {
+        const parentTypes = parentTypesOrEmpty(node);
+        if (isEmpty(parentTypes)) {
+            return;
+        }
+
+        const [firstParentType, ...otherParentTypes] = parentTypes;
+
+        // First parent type must be a class
+        const computedType = computeType(firstParentType);
+        if (computedType !== UnknownType && !(computedType instanceof ClassType)) {
+            accept('error', 'A class must only inherit classes.', {
+                node: firstParentType,
+                code: CODE_INHERITANCE_NOT_A_CLASS,
+            });
+        }
+
+        // Must have only one parent type
+        for (const parentType of otherParentTypes) {
+            accept('error', 'Multiple inheritance is not supported. Only the first parent type will be considered.', {
+                node: parentType,
+                code: CODE_INHERITANCE_MULTIPLE_INHERITANCE,
+            });
+        }
+    };
+};
+
+export const classMustNotInheritItself = (services: SafeDsServices) => {
+    const classHierarchy = services.types.ClassHierarchy;
+
+    return (node: SdsClass, accept: ValidationAcceptor): void => {
+        const superClasses = classHierarchy.streamSuperclasses(node);
+        if (superClasses.includes(node)) {
+            accept('error', 'A class must not directly or indirectly be a subtype of itself.', {
+                node: parentTypesOrEmpty(node)[0],
+                code: CODE_INHERITANCE_CYCLE,
+            });
+        }
+    };
+};

src/language/validation/inheritance.ts

@@ -71,7 +71,7 @@ import {
 import { placeholderShouldBeUsed, placeholdersMustNotBeAnAlias } from './other/declarations/placeholders.js';
 import { segmentParameterShouldBeUsed, segmentResultMustBeAssignedExactlyOnce } from './other/declarations/segments.js';
 import { lambdaParameterMustNotHaveConstModifier } from './other/expressions/lambdas.js';
-import { indexedAccessesShouldBeUsedWithCaution } from './experimentalLanguageFeature.js';
+import { indexedAccessesShouldBeUsedWithCaution } from './experimentalLanguageFeatures.js';
 import { requiredParameterMustNotBeExpert } from './builtins/expert.js';
 import {
     callableTypeParametersMustNotBeAnnotated,
@@ -86,6 +86,7 @@ import {
     namedTypeMustNotSetTypeParameterMultipleTimes,
     namedTypeTypeArgumentListMustNotHavePositionalArgumentsAfterNamedArguments,
 } from './other/types/namedTypes.js';
+import { classMustNotInheritItself, classMustOnlyInheritASingleClass } from './inheritance.js';
 
 /**
  * Register custom validation checks.
@@ -124,7 +125,11 @@ export const registerValidationChecks = function (services: SafeDsServices) {
             callableTypeParameterMustNotHaveConstModifier,
             callableTypeResultsMustNotBeAnnotated,
         ],
-        SdsClass: [classMustContainUniqueNames],
+        SdsClass: [
+            classMustContainUniqueNames,
+            classMustOnlyInheritASingleClass(services),
+            classMustNotInheritItself(services),
+        ],
         SdsClassBody: [classBodyShouldNotBeEmpty],
         SdsConstraintList: [constraintListShouldNotBeEmpty],
         SdsDeclaration: [nameMustNotStartWithBlockLambdaPrefix, nameShouldHaveCorrectCasing],

src/language/validation/safe-ds-validator.ts

@@ -36,7 +36,7 @@ describe('pluralize', () => {
             singular: 'apple',
             expected: 'apples',
         },
-    ])('should return the singular or plural form based on the count', ({ count, singular, plural, expected }) => {
+    ])('should return the singular or plural form based on the count (%#)', ({ count, singular, plural, expected }) => {
         expect(pluralize(count, singular, plural)).toBe(expected);
     });
 });

tests/language/helpers/stringUtils.test.ts

@@ -0,0 +1,88 @@
+import { afterEach, beforeEach, describe, expect, it } from 'vitest';
+import { createSafeDsServices } from '../../../src/language/safe-ds-module.js';
+import { NodeFileSystem } from 'langium/node';
+import { clearDocuments } from 'langium/test';
+import { isSdsClass, SdsClass } from '../../../src/language/generated/ast.js';
+import { getNodeOfType } from '../../helpers/nodeFinder.js';
+
+const services = createSafeDsServices(NodeFileSystem).SafeDs;
+const classHierarchy = services.types.ClassHierarchy;
+
+describe('SafeDsClassHierarchy', async () => {
+    beforeEach(async () => {
+        // Load the builtin library
+        await services.shared.workspace.WorkspaceManager.initializeWorkspace([]);
+    });
+
+    afterEach(async () => {
+        await clearDocuments(services);
+    });
+
+    describe('streamSuperclasses', () => {
+        const superclassNames = (node: SdsClass | undefined) =>
+            classHierarchy
+                .streamSuperclasses(node)
+                .map((clazz) => clazz.name)
+                .toArray();
+
+        it('should return an empty stream if passed undefined', () => {
+            expect(superclassNames(undefined)).toStrictEqual([]);
+        });
+
+        const testCases = [
+            {
+                testName: 'should return "Any" if the class has no parent types',
+                code: `
+                    class A
+                `,
+                expected: ['Any'],
+            },
+            {
+                testName: 'should return "Any" if the first parent type is not a class',
+                code: `
+                    class A sub E
+                    enum E
+                `,
+                expected: ['Any'],
+            },
+            {
+                testName: 'should return the superclasses of a class (no cycle, implicit any)',
+                code: `
+                    class A sub B
+                    class B
+                `,
+                expected: ['B', 'Any'],
+            },
+            {
+                testName: 'should return the superclasses of a class (no cycle, explicit any)',
+                code: `
+                    class A sub Any
+                `,
+                expected: ['Any'],
+            },
+            {
+                testName: 'should return the superclasses of a class (cycle)',
+                code: `
+                    class A sub B
+                    class B sub C
+                    class C sub A
+                `,
+                expected: ['B', 'C', 'A', 'Any'],
+            },
+            {
+                testName: 'should only consider the first parent type',
+                code: `
+                    class A sub B, C
+                    class B
+                    class C
+                `,
+                expected: ['B', 'Any'],
+            },
+        ];
+
+        it.each(testCases)('$testName', async ({ code, expected }) => {
+            const firstClass = await getNodeOfType(services, code, isSdsClass);
+            expect(superclassNames(firstClass)).toStrictEqual(expected);
+        });
+    });
+});

tests/language/typing/safe-ds-class-hierarchy.test.ts

@@ -0,0 +1,22 @@
+package tests.validation.inheritance.mustBeAcyclic
+
+// $TEST$ error "A class must not directly or indirectly be a subtype of itself."
+class MyClass1 sub »MyClass3«
+// $TEST$ error "A class must not directly or indirectly be a subtype of itself."
+class MyClass2 sub »MyClass1«
+// $TEST$ error "A class must not directly or indirectly be a subtype of itself."
+class MyClass3 sub »MyClass2«
+
+class MyClass4
+// $TEST$ no error "A class must not directly or indirectly be a subtype of itself."
+class MyClass5 sub »MyClass4«
+
+// $TEST$ no error "A class must not directly or indirectly be a subtype of itself."
+class MyClass6 sub »MyClass7«
+// $TEST$ no error "A class must not directly or indirectly be a subtype of itself."
+class MyClass7 sub Any, »MyClass6«
+
+// $TEST$ no error "A class must not directly or indirectly be a subtype of itself."
+class MyClass8 sub »Unresolved«
+// $TEST$ no error "A class must not directly or indirectly be a subtype of itself."
+class MyClass9 sub »MyClass8«