feat: allow literal types as upper bounds of type parameters (#1102)

Closes #1081

### Summary of Changes

* Literal types can now be used as upper bounds of type parameters
* Don't show an extra error if callable types or union types are used as
upper bounds of type parameters. This is already checked elsewhere.

packages/safe-ds-lang/src/language/validation/other/declarations/typeParameters.ts

@@ -17,7 +17,7 @@ import {
 import { isStatic, TypeParameter } from '../../../helpers/nodeProperties.js';
 import { SafeDsServices } from '../../../safe-ds-module.js';
 import { SafeDsNodeMapper } from '../../../helpers/safe-ds-node-mapper.js';
-import { NamedType, UnknownType } from '../../../typing/model.js';
+import { UnknownType } from '../../../typing/model.js';
 
 export const CODE_TYPE_PARAMETER_INSUFFICIENT_CONTEXT = 'type-parameter/insufficient-context';
 export const CODE_TYPE_PARAMETER_UPPER_BOUND = 'type-parameter/upper-bound';
@@ -66,14 +66,16 @@ export const typeParameterMustHaveSufficientContext = (node: SdsTypeParameter, a
     }
 };
 
-export const typeParameterUpperBoundMustBeNamedType = (services: SafeDsServices) => {
+export const typeParameterUpperBoundMustNotBeUnknown = (services: SafeDsServices) => {
     const typeComputer = services.typing.TypeComputer;
 
     return (node: SdsTypeParameter, accept: ValidationAcceptor) => {
-        const boundType = typeComputer.computeType(node.upperBound);
+        if (!node.upperBound) {
+            return;
+        }
 
-        if (boundType !== UnknownType && !(boundType instanceof NamedType)) {
-            accept('error', 'The upper bound of a type parameter must be a named type.', {
+        if (typeComputer.computeType(node.upperBound) === UnknownType) {
+            accept('error', 'The upper bound of a type parameter must not have an unknown type.', {
                 node,
                 property: 'upperBound',
                 code: CODE_TYPE_PARAMETER_UPPER_BOUND,

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

@@ -85,7 +85,7 @@ import {
     typeParameterMustBeUsedInCorrectPosition,
     typeParameterMustHaveSufficientContext,
     typeParameterMustOnlyBeVariantOnClass,
-    typeParameterUpperBoundMustBeNamedType,
+    typeParameterUpperBoundMustNotBeUnknown,
 } from './other/declarations/typeParameters.js';
 import { callArgumentMustBeConstantIfParameterIsConstant, callMustNotBeRecursive } from './other/expressions/calls.js';
 import { divisionDivisorMustNotBeZero } from './other/expressions/infixOperations.js';
@@ -377,7 +377,7 @@ export const registerValidationChecks = function (services: SafeDsServices) {
             typeParameterMustBeUsedInCorrectPosition(services),
             typeParameterMustHaveSufficientContext,
             typeParameterMustOnlyBeVariantOnClass,
-            typeParameterUpperBoundMustBeNamedType(services),
+            typeParameterUpperBoundMustNotBeUnknown(services),
         ],
         SdsTypeParameterList: [
             typeParameterListMustNotHaveRequiredTypeParametersAfterOptionalTypeParameters,

packages/safe-ds-lang/tests/resources/validation/other/declarations/type parameters/upper bound must not be unknown/main.sdsdev

@@ -0,0 +1,34 @@
+package tests.validation.other.declarations.typeParameters.upperBoundMustNotBeUnknown
+
+class C
+enum E {
+    V
+}
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass1<T sub »() -> ()«>
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass2<T sub »literal<1>«>
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass3<T sub »C«>
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass4<T sub »E«>
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass5<T sub »E.V«>
+
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass6<T sub »union<C, E>«>
+
+// $TEST$ error "The upper bound of a type parameter must not have an unknown type."
+// $TEST$ no error "The upper bound of a type parameter must not have an unknown type."
+class MyClass7<T1 sub »T2«, T2 sub »T1«>
+
+// $TEST$ error "The upper bound of a type parameter must not have an unknown type."
+class MyClass8<T sub »Unresolved«>
+
+// $TEST$ error "The upper bound of a type parameter must not have an unknown type."
+class MyClass9<T sub »unknown«>