feat: error if parent type is nullable (#891)

### Summary of Changes Only allow non-nullable class types as the parent types of a class.
Safe-DS · Feb 17, 2024 · add650d · add650d
1 parent bb0c94b
commit add650d
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Showing 6 changed files with 58 additions and 18 deletions.
diff --git a/packages/safe-ds-lang/src/language/typing/safe-ds-type-computer.ts b/packages/safe-ds-lang/src/language/typing/safe-ds-type-computer.ts
@@ -1110,17 +1110,15 @@ export class SafeDsTypeComputer {
         // Compared to `ClassHierarchy.properSuperclassesGenerator`, we already include the given type in the list of
         // visited elements, since this method here is not used to detect cycles.
         const visited = new Set<SdsClass>([type.declaration]);
-        let isNullable = type.isNullable;
         let current = this.parentClassType(type);
 
         while (current && !visited.has(current.declaration)) {
             yield current;
             visited.add(current.declaration);
-            isNullable ||= current.isNullable;
             current = this.parentClassType(current);
         }
 
-        const Any = this.coreTypes.Any.updateNullability(isNullable);
+        const Any = this.coreTypes.Any.updateNullability(type.isNullable);
         if (Any instanceof ClassType && !visited.has(Any.declaration)) {
             yield Any;
         }
@@ -1136,7 +1134,7 @@ export class SafeDsTypeComputer {
         const firstParentType = this.computeType(firstParentTypeNode, type.substitutions);
 
         if (firstParentType instanceof ClassType) {
-            return firstParentType.updateNullability(type.isNullable || firstParentType.isNullable);
+            return firstParentType.updateNullability(type.isNullable);
         }
         return undefined;
     }

diff --git a/packages/safe-ds-lang/src/language/validation/inheritance.ts b/packages/safe-ds-lang/src/language/validation/inheritance.ts
@@ -10,6 +10,7 @@ export const CODE_INHERITANCE_MULTIPLE_INHERITANCE = 'inheritance/multiple-inher
 export const CODE_INHERITANCE_IDENTICAL_TO_OVERRIDDEN_MEMBER = 'inheritance/identical-to-overridden-member';
 export const CODE_INHERITANCE_INCOMPATIBLE_TO_OVERRIDDEN_MEMBER = 'inheritance/incompatible-to-overridden-member';
 export const CODE_INHERITANCE_NOT_A_CLASS = 'inheritance/not-a-class';
+export const CODE_INHERITANCE_NULLABLE = 'inheritance/nullable';
 
 export const classMemberMustMatchOverriddenMemberAndShouldBeNeeded = (services: SafeDsServices) => {
     const builtinAnnotations = services.builtins.Annotations;
@@ -115,11 +116,18 @@ export const classMustOnlyInheritASingleClass = (services: SafeDsServices) => {
 
         // 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,
-            });
+        if (computedType !== UnknownType) {
+            if (!(computedType instanceof ClassType)) {
+                accept('error', 'A class must only inherit classes.', {
+                    node: firstParentType!,
+                    code: CODE_INHERITANCE_NOT_A_CLASS,
+                });
+            } else if (computedType.isNullable) {
+                accept('error', 'The parent type must not be nullable.', {
+                    node: firstParentType!,
+                    code: CODE_INHERITANCE_NULLABLE,
+                });
+            }
         }
 
         // Must have only one parent type

diff --git a/packages/safe-ds-lang/tests/language/typing/type computer/streamSupertypes.test.ts b/packages/safe-ds-lang/tests/language/typing/type computer/streamSupertypes.test.ts
@@ -106,7 +106,7 @@ describe('streamProperSupertypes', async () => {
             expected: ['B', 'Any'],
         },
         {
-            testName: 'should consider the nullability of the parent type',
+            testName: 'should disregard the nullability of the parent type',
             code: `
                     class Test {
                         attr a: A
@@ -115,7 +115,7 @@ describe('streamProperSupertypes', async () => {
                     class A sub B?
                     class B
             `,
-            expected: ['B?', 'Any?'],
+            expected: ['B', 'Any'],
         },
         {
             testName: 'should consider the nullability of the start type',

diff --git a/...esources/validation/inheritance/must inherit only classes/class with parent types.sdstest b/...esources/validation/inheritance/must inherit only classes/class with parent types.sdstest
@@ -12,23 +12,23 @@ enum MyEnum {
 // $TEST$ no error "A class must only inherit classes."
 // $TEST$ no error "A class must only inherit classes."
 // $TEST$ no error "A class must only inherit classes."
-class TestClass sub »MyClass«,
+class TestClass1 sub »MyClass«,
                     »MyClass.MyNestedClass«,
                     »MyEnum«,
                     »MyEnum.MyEnumVariant«,
                     »Unresolved«
 
 // $TEST$ no error "A class must only inherit classes."
-class TestClass sub »MyClass«
+class TestClass2 sub »MyClass«
 
 // $TEST$ no error "A class must only inherit classes."
-class TestClass sub »MyClass.MyNestedClass«
+class TestClass3 sub »MyClass.MyNestedClass«
 
 // $TEST$ error "A class must only inherit classes."
-class TestClass sub »MyEnum«
+class TestClass4 sub »MyEnum«
 
 // $TEST$ error "A class must only inherit classes."
-class TestClass sub »MyEnum.MyEnumVariant«
+class TestClass5 sub »MyEnum.MyEnumVariant«
 
 // $TEST$ no error "A class must only inherit classes."
-class TestClass sub »Unresolved«
+class TestClass6 sub »Unresolved«
diff --git a/...urces/validation/inheritance/must inherit only classes/class without parent types.sdstest b/...urces/validation/inheritance/must inherit only classes/class without parent types.sdstest
@@ -1,4 +1,4 @@
 package tests.validation.inheritance.mustInheritOnlyClasses
 
 // $TEST$ no error "A class must only inherit classes."
-class TestClass
+class TestClass7
diff --git a/...sts/resources/validation/inheritance/must not be nullable/class with parent types.sdstest b/...sts/resources/validation/inheritance/must not be nullable/class with parent types.sdstest
@@ -0,0 +1,34 @@
+package tests.validation.inheritance.mustNotBeNullable
+
+class MyClass {
+    class MyNestedClass
+}
+enum MyEnum {
+    MyEnumVariant
+}
+
+// $TEST$ no error "The parent type must not be nullable."
+// $TEST$ no error "The parent type must not be nullable."
+// $TEST$ no error "The parent type must not be nullable."
+// $TEST$ no error "The parent type must not be nullable."
+// $TEST$ no error "The parent type must not be nullable."
+class TestClass1 sub »MyClass«,
+                    »MyClass.MyNestedClass?«,
+                    »MyEnum?«,
+                    »MyEnum.MyEnumVariant?«,
+                    »Unresolved?«
+
+// $TEST$ error "The parent type must not be nullable."
+class TestClass2 sub »MyClass?«
+
+// $TEST$ error "The parent type must not be nullable."
+class TestClass3 sub »MyClass.MyNestedClass?«
+
+// $TEST$ no error "The parent type must not be nullable."
+class TestClass4 sub »MyEnum?«
+
+// $TEST$ no error "The parent type must not be nullable."
+class TestClass5 sub »MyEnum.MyEnumVariant?«
+
+// $TEST$ no error "The parent type must not be nullable."
+class TestClass6 sub »Unresolved?«