feat: compute type of elvis operators with nullable left operand (#715)

Closes #541.

### Summary of Changes

Add logic to compute the lowest common supertype of several types. This
is needed to compute the type of elvis operators with a nullable left
operand. It's also later needed for the inference of type parameters.

packages/safe-ds-lang/src/language/partialEvaluation/safe-ds-partial-evaluator.ts

@@ -1,21 +1,5 @@
-import { SafeDsServices } from '../safe-ds-module.js';
 import { AstNode, AstNodeLocator, getDocument, WorkspaceCache } from 'langium';
-import {
-    BooleanConstant,
-    EvaluatedEnumVariant,
-    EvaluatedList,
-    EvaluatedMap,
-    EvaluatedMapEntry,
-    EvaluatedNode,
-    FloatConstant,
-    IntConstant,
-    isConstant,
-    NullConstant,
-    NumberConstant,
-    ParameterSubstitutions,
-    StringConstant,
-    UnknownEvaluatedNode,
-} from './model.js';
+import { isEmpty } from '../../helpers/collectionUtils.js';
 import {
     isSdsArgument,
     isSdsBlockLambda,
@@ -55,7 +39,23 @@ import {
 } from '../generated/ast.js';
 import { getArguments, getParameters } from '../helpers/nodeProperties.js';
 import { SafeDsNodeMapper } from '../helpers/safe-ds-node-mapper.js';
-import { isEmpty } from '../../helpers/collectionUtils.js';
+import { SafeDsServices } from '../safe-ds-module.js';
+import {
+    BooleanConstant,
+    EvaluatedEnumVariant,
+    EvaluatedList,
+    EvaluatedMap,
+    EvaluatedMapEntry,
+    EvaluatedNode,
+    FloatConstant,
+    IntConstant,
+    isConstant,
+    NullConstant,
+    NumberConstant,
+    ParameterSubstitutions,
+    StringConstant,
+    UnknownEvaluatedNode,
+} from './model.js';
 
 export class SafeDsPartialEvaluator {
     private readonly astNodeLocator: AstNodeLocator;
@@ -141,7 +141,7 @@ export class SafeDsPartialEvaluator {
         } else if (isSdsTemplateString(node)) {
             return this.evaluateTemplateString(node, substitutions);
         } /* c8 ignore start */ else {
-            return UnknownEvaluatedNode;
+            throw new Error(`Unexpected node type: ${node.$type}`);
         } /* c8 ignore stop */
     }
 
@@ -269,7 +269,7 @@ export class SafeDsPartialEvaluator {
 
             /* c8 ignore next 2 */
             default:
-                return UnknownEvaluatedNode;
+                throw new Error(`Unexpected operator: ${node.operator}`);
         }
     }
 

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

@@ -1,3 +1,4 @@
+import { isEmpty } from '../../helpers/collectionUtils.js';
 import {
     SdsAbstractResult,
     SdsCallable,
@@ -29,19 +30,14 @@ export abstract class Type {
     abstract toString(): string;
 
     /**
-     * Returns a copy of this type with the given nullability. If nullability is fixed for a type, it may return the
-     * instance itself.
+     * Removes any unnecessary containers from the type.
      */
-    updateNullability(_isNullable: boolean): Type {
-        return this;
-    }
+    abstract unwrap(): Type;
 
     /**
-     * Removes any unnecessary containers from the type.
+     * Returns a copy of this type with the given nullability.
      */
-    unwrap(): Type {
-        return this;
-    }
+    abstract updateNullability(isNullable: boolean): Type;
 }
 
 export class CallableType extends Type {
@@ -79,14 +75,32 @@ export class CallableType extends Type {
     override toString(): string {
         return `${this.inputType} -> ${this.outputType}`;
     }
+
+    override unwrap(): CallableType {
+        return new CallableType(
+            this.callable,
+            new NamedTupleType(...this.inputType.entries.map((it) => it.unwrap())),
+            new NamedTupleType(...this.outputType.entries.map((it) => it.unwrap())),
+        );
+    }
+
+    override updateNullability(isNullable: boolean): Type {
+        if (!isNullable) {
+            return this;
+        }
+
+        return new UnionType(this, new LiteralType(NullConstant));
+    }
 }
 
 export class LiteralType extends Type {
+    readonly constants: Constant[];
     override readonly isNullable: boolean;
 
-    constructor(readonly constants: Constant[]) {
+    constructor(...constants: Constant[]) {
         super();
 
+        this.constants = constants;
         this.isNullable = constants.some((it) => it === NullConstant);
     }
 
@@ -107,28 +121,39 @@ export class LiteralType extends Type {
         return `literal<${this.constants.join(', ')}>`;
     }
 
+    override unwrap(): LiteralType {
+        return this;
+    }
+
     override updateNullability(isNullable: boolean): LiteralType {
         if (this.isNullable && !isNullable) {
-            return new LiteralType(this.constants.filter((it) => it !== NullConstant));
+            return new LiteralType(...this.constants.filter((it) => it !== NullConstant));
         } else if (!this.isNullable && isNullable) {
-            return new LiteralType([...this.constants, NullConstant]);
+            return new LiteralType(...this.constants, NullConstant);
         } else {
             return this;
         }
     }
 }
 
 export class NamedTupleType<T extends SdsDeclaration> extends Type {
+    readonly entries: NamedTupleEntry<T>[];
     override readonly isNullable = false;
 
-    constructor(readonly entries: NamedTupleEntry<T>[]) {
+    constructor(...entries: NamedTupleEntry<T>[]) {
         super();
+
+        this.entries = entries;
     }
 
     /**
      * The length of this tuple.
      */
-    readonly length: number = this.entries.length;
+    /* c8 ignore start */
+    get length(): number {
+        return this.entries.length;
+    }
+    /* c8 ignore stop */
 
     /**
      * Returns the type of the entry at the given index. If the index is out of bounds, returns `undefined`.
@@ -159,10 +184,18 @@ export class NamedTupleType<T extends SdsDeclaration> extends Type {
      */
     override unwrap(): Type {
         if (this.entries.length === 1) {
-            return this.entries[0].type;
+            return this.entries[0].type.unwrap();
         }
 
-        return this;
+        return new NamedTupleType(...this.entries.map((it) => it.unwrap()));
+    }
+
+    override updateNullability(isNullable: boolean): Type {
+        if (!isNullable) {
+            return this;
+        }
+
+        return new UnionType(this, new LiteralType(NullConstant));
     }
 }
 
@@ -180,6 +213,10 @@ export class NamedTupleEntry<T extends SdsDeclaration> {
     toString(): string {
         return `${this.name}: ${this.type}`;
     }
+
+    unwrap(): NamedTupleEntry<T> {
+        return new NamedTupleEntry(this.declaration, this.name, this.type.unwrap());
+    }
 }
 
 export abstract class NamedType<T extends SdsDeclaration> extends Type {
@@ -196,6 +233,10 @@ export abstract class NamedType<T extends SdsDeclaration> extends Type {
     }
 
     abstract override updateNullability(isNullable: boolean): NamedType<T>;
+
+    unwrap(): NamedType<T> {
+        return this;
+    }
 }
 
 export class ClassType extends NamedType<SdsClass> {
@@ -290,14 +331,28 @@ export class StaticType extends Type {
     override toString(): string {
         return `$type<${this.instanceType}>`;
     }
+
+    override unwrap(): Type {
+        return this;
+    }
+
+    override updateNullability(isNullable: boolean): Type {
+        if (!isNullable) {
+            return this;
+        }
+
+        return new UnionType(this, new LiteralType(NullConstant));
+    }
 }
 
 export class UnionType extends Type {
+    readonly possibleTypes: Type[];
     override readonly isNullable: boolean;
 
-    constructor(readonly possibleTypes: Type[]) {
+    constructor(...possibleTypes: Type[]) {
         super();
 
+        this.possibleTypes = possibleTypes;
         this.isNullable = possibleTypes.some((it) => it.isNullable);
     }
 
@@ -317,6 +372,28 @@ export class UnionType extends Type {
     override toString(): string {
         return `union<${this.possibleTypes.join(', ')}>`;
     }
+
+    override unwrap(): Type {
+        if (this.possibleTypes.length === 1) {
+            return this.possibleTypes[0].unwrap();
+        }
+
+        return new UnionType(...this.possibleTypes.map((it) => it.unwrap()));
+    }
+
+    override updateNullability(isNullable: boolean): Type {
+        if (this.isNullable && !isNullable) {
+            return new UnionType(...this.possibleTypes.map((it) => it.updateNullability(false)));
+        } else if (!this.isNullable && isNullable) {
+            if (isEmpty(this.possibleTypes)) {
+                return new LiteralType(NullConstant);
+            } else {
+                return new UnionType(...this.possibleTypes.map((it) => it.updateNullability(true)));
+            }
+        } else {
+            return this;
+        }
+    }
 }
 
 class UnknownTypeClass extends Type {
@@ -329,22 +406,14 @@ class UnknownTypeClass extends Type {
     override toString(): string {
         return '?';
     }
-}
-
-export const UnknownType = new UnknownTypeClass();
 
-/* c8 ignore start */
-class NotImplementedTypeClass extends Type {
-    override readonly isNullable = false;
-
-    override equals(other: Type): boolean {
-        return other instanceof NotImplementedTypeClass;
+    override unwrap(): Type {
+        return this;
     }
 
-    override toString(): string {
-        return '$NotImplemented';
+    override updateNullability(_isNullable: boolean): Type {
+        return this;
     }
 }
 
-export const NotImplementedType = new NotImplementedTypeClass();
-/* c8 ignore stop */
+export const UnknownType = new UnknownTypeClass();

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

@@ -1,23 +1,19 @@
 import { EMPTY_STREAM, stream, Stream } from 'langium';
 import { SafeDsClasses } from '../builtins/safe-ds-classes.js';
-import { SdsClass, type SdsClassMember } from '../generated/ast.js';
+import { isSdsClass, isSdsNamedType, SdsClass, type SdsClassMember } from '../generated/ast.js';
 import { getMatchingClassMembers, getParentTypes } from '../helpers/nodeProperties.js';
 import { SafeDsServices } from '../safe-ds-module.js';
-import { ClassType } from './model.js';
-import { SafeDsTypeComputer } from './safe-ds-type-computer.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 `true` if the given node is equal to or a subclass of the given other node. If one of the nodes is
-     * undefined, `false` is returned.
+     * `undefined`, `false` is returned.
      */
     isEqualToOrSubclassOf(node: SdsClass | undefined, other: SdsClass | undefined): boolean {
         if (!node || !other) {
@@ -74,9 +70,11 @@ export class SafeDsClassHierarchy {
      */
     private parentClassOrUndefined(node: SdsClass | undefined): SdsClass | undefined {
         const [firstParentType] = getParentTypes(node);
-        const computedType = this.typeComputer.computeType(firstParentType);
-        if (computedType instanceof ClassType) {
-            return computedType.declaration;
+        if (isSdsNamedType(firstParentType)) {
+            const declaration = firstParentType.declaration?.ref;
+            if (isSdsClass(declaration)) {
+                return declaration;
+            }
         }
 
         return undefined;

packages/safe-ds-lang/src/language/typing/safe-ds-core-types.ts

@@ -13,6 +13,10 @@ export class SafeDsCoreTypes {
         this.cache = new WorkspaceCache(services.shared);
     }
 
+    get Any(): Type {
+        return this.createCoreType(this.builtinClasses.Any, false);
+    }
+
     get AnyOrNull(): Type {
         return this.createCoreType(this.builtinClasses.Any, true);
     }
@@ -37,11 +41,13 @@ export class SafeDsCoreTypes {
         return this.createCoreType(this.builtinClasses.Map);
     }
 
-    /* c8 ignore start */
+    get Nothing(): Type {
+        return this.createCoreType(this.builtinClasses.Nothing, false);
+    }
+
     get NothingOrNull(): Type {
         return this.createCoreType(this.builtinClasses.Nothing, true);
     }
-    /* c8 ignore stop */
 
     get String(): Type {
         return this.createCoreType(this.builtinClasses.String);