feat: ensure an overriding member matches the overridden one (#758)

Closes #639

### Summary of Changes

Show an error if an overriding member does not match the overridden one.

packages/safe-ds-lang/src/language/generation/safe-ds-python-generator.ts

@@ -1,4 +1,26 @@
-import { SafeDsServices } from '../safe-ds-module.js';
+import {
+    CompositeGeneratorNode,
+    expandToNode,
+    expandTracedToNode,
+    findRootNode,
+    getContainerOfType,
+    getDocument,
+    joinToNode,
+    joinTracedToNode,
+    LangiumDocument,
+    NL,
+    streamAllContents,
+    toStringAndTrace,
+    TraceRegion,
+    traceToNode,
+    TreeStreamImpl,
+    URI,
+} from 'langium';
+import path from 'path';
+import { SourceMapGenerator, StartOfSourceMap } from 'source-map';
+import { TextDocument } from 'vscode-languageserver-textdocument';
+import { groupBy } from '../../helpers/collectionUtils.js';
+import { SafeDsAnnotations } from '../builtins/safe-ds-annotations.js';
 import {
     isSdsAbstractResult,
     isSdsAssignment,
@@ -46,49 +68,27 @@ import {
     SdsStatement,
 } from '../generated/ast.js';
 import { isInStubFile, isStubFile } from '../helpers/fileExtensions.js';
-import path from 'path';
-import {
-    CompositeGeneratorNode,
-    expandToNode,
-    expandTracedToNode,
-    findRootNode,
-    getContainerOfType,
-    getDocument,
-    joinToNode,
-    joinTracedToNode,
-    LangiumDocument,
-    NL,
-    streamAllContents,
-    toStringAndTrace,
-    TraceRegion,
-    traceToNode,
-    TreeStreamImpl,
-    URI,
-} from 'langium';
+import { IdManager } from '../helpers/idManager.js';
 import {
     getAbstractResults,
     getAssignees,
     getImportedDeclarations,
     getImports,
     getModuleMembers,
     getStatements,
-    isRequiredParameter,
+    Parameter,
     streamBlockLambdaResults,
 } from '../helpers/nodeProperties.js';
-import { groupBy } from '../../helpers/collectionUtils.js';
+import { SafeDsNodeMapper } from '../helpers/safe-ds-node-mapper.js';
 import {
     BooleanConstant,
     FloatConstant,
     IntConstant,
     NullConstant,
     StringConstant,
 } from '../partialEvaluation/model.js';
-import { IdManager } from '../helpers/idManager.js';
-import { TextDocument } from 'vscode-languageserver-textdocument';
-import { SafeDsAnnotations } from '../builtins/safe-ds-annotations.js';
-import { SafeDsNodeMapper } from '../helpers/safe-ds-node-mapper.js';
 import { SafeDsPartialEvaluator } from '../partialEvaluation/safe-ds-partial-evaluator.js';
-import { SourceMapGenerator, StartOfSourceMap } from 'source-map';
+import { SafeDsServices } from '../safe-ds-module.js';
 
 export const CODEGEN_PREFIX = '__gen_';
 const BLOCK_LAMBDA_PREFIX = `${CODEGEN_PREFIX}block_lambda_`;
@@ -685,7 +685,7 @@ export class SafeDsPythonGenerator {
     private generateArgument(argument: SdsArgument, frame: GenerationInfoFrame): CompositeGeneratorNode {
         const parameter = this.nodeMapper.argumentToParameter(argument);
         return expandTracedToNode(argument)`${
-            parameter !== undefined && !isRequiredParameter(parameter)
+            parameter !== undefined && !Parameter.isRequired(parameter)
                 ? expandToNode`${this.generateParameter(parameter, frame, false)}=`
                 : ''
         }${this.generateExpression(argument.value, frame)}`;

packages/safe-ds-lang/src/language/helpers/nodeProperties.ts

@@ -16,6 +16,7 @@ import {
     isSdsLambda,
     isSdsModule,
     isSdsModuleMember,
+    isSdsParameter,
     isSdsPlaceholder,
     isSdsSegment,
     isSdsTypeParameterList,
@@ -85,28 +86,30 @@ export const isPositionalArgument = (node: SdsArgument): boolean => {
     return !node.parameter;
 };
 
-export const isNamedTypeArgument = (node: SdsTypeArgument): boolean => {
-    return Boolean(node.typeParameter);
-};
+export namespace Parameter {
+    export const isConstant = (node: SdsParameter | undefined): boolean => {
+        if (!node) {
+            return false;
+        }
 
-export const isConstantParameter = (node: SdsParameter | undefined): boolean => {
-    if (!node) {
-        return false;
-    }
+        const containingCallable = getContainerOfType(node, isSdsCallable);
 
-    const containingCallable = getContainerOfType(node, isSdsCallable);
+        // In those cases, the const modifier is not applicable
+        if (isSdsCallableType(containingCallable) || isSdsLambda(containingCallable)) {
+            return false;
+        }
 
-    // In those cases, the const modifier is not applicable
-    if (isSdsCallableType(containingCallable) || isSdsLambda(containingCallable)) {
-        return false;
-    }
+        return isSdsAnnotation(containingCallable) || node.isConstant;
+    };
 
-    return isSdsAnnotation(containingCallable) || node.isConstant;
-};
+    export const isOptional = (node: SdsParameter | undefined): boolean => {
+        return Boolean(node?.defaultValue);
+    };
 
-export const isRequiredParameter = (node: SdsParameter): boolean => {
-    return !node.defaultValue;
-};
+    export const isRequired = (node: SdsParameter | undefined): boolean => {
+        return isSdsParameter(node) && !node.defaultValue;
+    };
+}
 
 export const isStatic = (node: SdsClassMember): boolean => {
     if (isSdsClass(node) || isSdsEnum(node)) {
@@ -121,6 +124,10 @@ export const isStatic = (node: SdsClassMember): boolean => {
     }
 };
 
+export const isNamedTypeArgument = (node: SdsTypeArgument): boolean => {
+    return Boolean(node.typeParameter);
+};
+
 // -------------------------------------------------------------------------------------------------
 // Accessors for list elements
 // -------------------------------------------------------------------------------------------------
@@ -190,11 +197,8 @@ export const streamBlockLambdaResults = (node: SdsBlockLambda | undefined): Stre
         .filter(isSdsBlockLambdaResult);
 };
 
-export const getMatchingClassMembers = (
-    node: SdsClass | undefined,
-    filterFunction: (member: SdsClassMember) => boolean = () => true,
-): SdsClassMember[] => {
-    return node?.body?.members?.filter(filterFunction) ?? [];
+export const getClassMembers = (node: SdsClass | undefined): SdsClassMember[] => {
+    return node?.body?.members ?? [];
 };
 
 export const getColumns = (node: SdsSchema | undefined): SdsColumn[] => {

packages/safe-ds-lang/src/language/scoping/safe-ds-scope-provider.ts

@@ -57,10 +57,10 @@ import {
     getAbstractResults,
     getAnnotationCallTarget,
     getAssignees,
+    getClassMembers,
     getEnumVariants,
     getImportedDeclarations,
     getImports,
-    getMatchingClassMembers,
     getPackageName,
     getParameters,
     getResults,
@@ -185,7 +185,7 @@ export class SafeDsScopeProvider extends DefaultScopeProvider {
         // Static access
         const declaration = this.getUniqueReferencedDeclarationForExpression(node.receiver);
         if (isSdsClass(declaration)) {
-            const ownStaticMembers = getMatchingClassMembers(declaration, isStatic);
+            const ownStaticMembers = getClassMembers(declaration).filter(isStatic);
             const superclassStaticMembers = this.classHierarchy.streamSuperclassMembers(declaration).filter(isStatic);
 
             return this.createScopeForNodes(ownStaticMembers, this.createScopeForNodes(superclassStaticMembers));
@@ -215,7 +215,7 @@ export class SafeDsScopeProvider extends DefaultScopeProvider {
         }
 
         if (receiverType instanceof ClassType) {
-            const ownInstanceMembers = getMatchingClassMembers(receiverType.declaration, (it) => !isStatic(it));
+            const ownInstanceMembers = getClassMembers(receiverType.declaration).filter((it) => !isStatic(it));
             const superclassInstanceMembers = this.classHierarchy
                 .streamSuperclassMembers(receiverType.declaration)
                 .filter((it) => !isStatic(it));

packages/safe-ds-lang/src/language/typing/model.ts

@@ -8,6 +8,7 @@ import {
     SdsEnumVariant,
     SdsParameter,
 } from '../generated/ast.js';
+import { Parameter } from '../helpers/nodeProperties.js';
 import { Constant, NullConstant } from '../partialEvaluation/model.js';
 
 /**
@@ -73,7 +74,11 @@ export class CallableType extends Type {
     }
 
     override toString(): string {
-        return `${this.inputType} -> ${this.outputType}`;
+        const inputTypeString = this.inputType.entries
+            .map((it) => `${it.name}${Parameter.isOptional(it.declaration) ? '?' : ''}: ${it.type}`)
+            .join(', ');
+
+        return `(${inputTypeString}) -> ${this.outputType}`;
     }
 
     override unwrap(): CallableType {

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

@@ -1,7 +1,7 @@
-import { EMPTY_STREAM, stream, Stream } from 'langium';
+import { EMPTY_STREAM, getContainerOfType, stream, Stream } from 'langium';
 import { SafeDsClasses } from '../builtins/safe-ds-classes.js';
 import { isSdsClass, isSdsNamedType, SdsClass, type SdsClassMember } from '../generated/ast.js';
-import { getMatchingClassMembers, getParentTypes } from '../helpers/nodeProperties.js';
+import { getClassMembers, getParentTypes, isStatic } from '../helpers/nodeProperties.js';
 import { SafeDsServices } from '../safe-ds-module.js';
 
 export class SafeDsClassHierarchy {
@@ -61,7 +61,7 @@ export class SafeDsClassHierarchy {
             return EMPTY_STREAM;
         }
 
-        return this.streamSuperclasses(node).flatMap(getMatchingClassMembers);
+        return this.streamSuperclasses(node).flatMap(getClassMembers);
     }
 
     /**
@@ -79,4 +79,31 @@ export class SafeDsClassHierarchy {
 
         return undefined;
     }
+
+    /**
+     * Returns the member that is overridden by the given member, or `undefined` if the member does not override
+     * anything.
+     */
+    getOverriddenMember(node: SdsClassMember | undefined): SdsClassMember | undefined {
+        // Static members cannot override anything
+        if (!node || isStatic(node)) {
+            return undefined;
+        }
+
+        // Don't consider members with the same name as a previous member
+        const containingClass = getContainerOfType(node, isSdsClass);
+        if (!containingClass) {
+            return undefined;
+        }
+        const firstMemberWithSameName = getClassMembers(containingClass).find(
+            (it) => !isStatic(it) && it.name === node.name,
+        );
+        if (firstMemberWithSameName !== node) {
+            return undefined;
+        }
+
+        return this.streamSuperclassMembers(containingClass)
+            .filter((it) => !isStatic(it) && it.name === node.name)
+            .head();
+    }
 }

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

@@ -1,7 +1,7 @@
 import { getContainerOfType } from 'langium';
 import type { SafeDsClasses } from '../builtins/safe-ds-classes.js';
 import { isSdsEnum, type SdsAbstractResult, SdsDeclaration } from '../generated/ast.js';
-import { getParameters } from '../helpers/nodeProperties.js';
+import { getParameters, Parameter } from '../helpers/nodeProperties.js';
 import { Constant } from '../partialEvaluation/model.js';
 import { SafeDsServices } from '../safe-ds-module.js';
 import {
@@ -84,6 +84,11 @@ export class SafeDsTypeChecker {
                     return false;
                 }
 
+                // Optionality must match (all but required to optional is OK)
+                if (Parameter.isRequired(typeEntry.declaration) && Parameter.isOptional(otherEntry.declaration)) {
+                    return false;
+                }
+
                 // Types must be contravariant
                 if (!this.isAssignableTo(otherEntry.type, typeEntry.type)) {
                     return false;
@@ -93,7 +98,7 @@ export class SafeDsTypeChecker {
             // Additional parameters must be optional
             for (let i = other.inputType.length; i < type.inputType.length; i++) {
                 const typeEntry = type.inputType.entries[i]!;
-                if (!typeEntry.declaration?.defaultValue) {
+                if (!Parameter.isOptional(typeEntry.declaration)) {
                     return false;
                 }
             }

packages/safe-ds-lang/src/language/validation/builtins/deprecated.ts

@@ -1,4 +1,5 @@
 import { ValidationAcceptor } from 'langium';
+import { DiagnosticTag } from 'vscode-languageserver';
 import {
     isSdsParameter,
     isSdsResult,
@@ -10,10 +11,9 @@ import {
     SdsParameter,
     SdsReference,
 } from '../../generated/ast.js';
+import { Parameter } from '../../helpers/nodeProperties.js';
 import { SafeDsServices } from '../../safe-ds-module.js';
-import { isRequiredParameter } from '../../helpers/nodeProperties.js';
 import { parameterCanBeAnnotated } from '../other/declarations/annotationCalls.js';
-import { DiagnosticTag } from 'vscode-languageserver';
 
 export const CODE_DEPRECATED_ASSIGNED_RESULT = 'deprecated/assigned-result';
 export const CODE_DEPRECATED_CALLED_ANNOTATION = 'deprecated/called-annotation';
@@ -108,7 +108,7 @@ export const referenceTargetShouldNotBeDeprecated =
 
 export const requiredParameterMustNotBeDeprecated =
     (services: SafeDsServices) => (node: SdsParameter, accept: ValidationAcceptor) => {
-        if (isRequiredParameter(node) && parameterCanBeAnnotated(node)) {
+        if (Parameter.isRequired(node) && parameterCanBeAnnotated(node)) {
             if (services.builtins.Annotations.isDeprecated(node)) {
                 accept('error', 'A deprecated parameter must be optional.', {
                     node,

packages/safe-ds-lang/src/language/validation/builtins/expert.ts

@@ -1,14 +1,14 @@
 import { ValidationAcceptor } from 'langium';
 import { SdsParameter } from '../../generated/ast.js';
+import { Parameter } from '../../helpers/nodeProperties.js';
 import { SafeDsServices } from '../../safe-ds-module.js';
-import { isRequiredParameter } from '../../helpers/nodeProperties.js';
 import { parameterCanBeAnnotated } from '../other/declarations/annotationCalls.js';
 
 export const CODE_EXPERT_TARGET_PARAMETER = 'expert/target-parameter';
 
 export const requiredParameterMustNotBeExpert =
     (services: SafeDsServices) => (node: SdsParameter, accept: ValidationAcceptor) => {
-        if (isRequiredParameter(node) && parameterCanBeAnnotated(node)) {
+        if (Parameter.isRequired(node) && parameterCanBeAnnotated(node)) {
             if (services.builtins.Annotations.isExpert(node)) {
                 accept('error', 'An expert parameter must be optional.', {
                     node,

packages/safe-ds-lang/src/language/validation/inheritance.ts

@@ -1,14 +1,47 @@
-import { ValidationAcceptor } from 'langium';
-import { SdsClass } from '../generated/ast.js';
+import { expandToStringWithNL, ValidationAcceptor } from 'langium';
+import { isEmpty } from '../../helpers/collectionUtils.js';
+import { SdsClass, type SdsClassMember } from '../generated/ast.js';
 import { getParentTypes } from '../helpers/nodeProperties.js';
 import { SafeDsServices } from '../safe-ds-module.js';
 import { ClassType, UnknownType } from '../typing/model.js';
-import { isEmpty } from '../../helpers/collectionUtils.js';
 
 export const CODE_INHERITANCE_CYCLE = 'inheritance/cycle';
 export const CODE_INHERITANCE_MULTIPLE_INHERITANCE = 'inheritance/multiple-inheritance';
+export const CODE_INHERITANCE_MUST_MATCH_OVERRIDDEN_MEMBER = 'inheritance/must-match-overridden-member';
 export const CODE_INHERITANCE_NOT_A_CLASS = 'inheritance/not-a-class';
 
+export const classMemberMustMatchOverriddenMember = (services: SafeDsServices) => {
+    const classHierarchy = services.types.ClassHierarchy;
+    const typeChecker = services.types.TypeChecker;
+    const typeComputer = services.types.TypeComputer;
+
+    return (node: SdsClassMember, accept: ValidationAcceptor): void => {
+        const overriddenMember = classHierarchy.getOverriddenMember(node);
+        if (!overriddenMember) {
+            return;
+        }
+
+        const ownMemberType = typeComputer.computeType(node);
+        const overriddenMemberType = typeComputer.computeType(overriddenMember);
+
+        if (!typeChecker.isAssignableTo(ownMemberType, overriddenMemberType)) {
+            accept(
+                'error',
+                expandToStringWithNL`
+                    Overriding member does not match the overridden member:
+                    - Expected type: ${overriddenMemberType}
+                    - Actual type:   ${ownMemberType}
+                `,
+                {
+                    node,
+                    property: 'name',
+                    code: CODE_INHERITANCE_MUST_MATCH_OVERRIDDEN_MEMBER,
+                },
+            );
+        }
+    };
+};
+
 export const classMustOnlyInheritASingleClass = (services: SafeDsServices) => {
     const typeComputer = services.types.TypeComputer;
     const computeType = typeComputer.computeType.bind(typeComputer);