From 5f7d8b9685df150d2fb030ba523566d6e194ca39 Mon Sep 17 00:00:00 2001
From: Lars Reimann <mail@larsreimann.com>
Date: Tue, 26 Mar 2024 11:30:47 +0100
Subject: [PATCH] feat: allow any widening/narrowing type cast

---
 .../src/language/validation/types.ts          | 12 +++++++++--
 .../types/checking/type casts/main.sdstest    | 21 +++++++++++++++----
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/packages/safe-ds-lang/src/language/validation/types.ts b/packages/safe-ds-lang/src/language/validation/types.ts
index c4be233ff..c9c70a572 100644
--- a/packages/safe-ds-lang/src/language/validation/types.ts
+++ b/packages/safe-ds-lang/src/language/validation/types.ts
@@ -354,12 +354,20 @@ export const prefixOperationOperandMustHaveCorrectType = (services: SafeDsServic
 };
 
 export const typeCastExpressionMustHaveUnknownType = (services: SafeDsServices) => {
+    const typeChecker = services.types.TypeChecker;
     const typeComputer = services.types.TypeComputer;
 
     return (node: SdsTypeCast, accept: ValidationAcceptor): void => {
         const expressionType = typeComputer.computeType(node.expression);
-        if (node.expression && expressionType !== UnknownType) {
-            accept('error', 'Type casts can only be applied to expressions of unknown type.', {
+        const targetType = typeComputer.computeType(node.type);
+
+        if (
+            node.expression &&
+            expressionType !== UnknownType &&
+            !typeChecker.isSubtypeOf(expressionType, targetType) &&
+            !typeChecker.isSupertypeOf(expressionType, targetType)
+        ) {
+            accept('error', 'This type cast can never succeed.', {
                 // Using property: "expression" does not work here, probably due to eclipse-langium/langium#1218
                 node: node.expression,
                 code: CODE_TYPE_MISMATCH,
diff --git a/packages/safe-ds-lang/tests/resources/validation/types/checking/type casts/main.sdstest b/packages/safe-ds-lang/tests/resources/validation/types/checking/type casts/main.sdstest
index 10308aa48..1676dd928 100644
--- a/packages/safe-ds-lang/tests/resources/validation/types/checking/type casts/main.sdstest	
+++ b/packages/safe-ds-lang/tests/resources/validation/types/checking/type casts/main.sdstest	
@@ -1,9 +1,22 @@
 package tests.validation.types.checking.typeCasts
 
+class C()
+class D() sub C
+class E() sub C
+
 pipeline test {
-    // $TEST$ no error "Type casts can only be applied to expressions of unknown type."
-    »unresolved« as Int;
+    // $TEST$ error "This type cast can never succeed."
+    »D()« as E;
+
+    // $TEST$ no error "This type cast can never succeed."
+    »C()« as C;
 
-    // $TEST$ error "Type casts can only be applied to expressions of unknown type."
-    »1« as Int;
+    // $TEST$ no error "This type cast can never succeed."
+    »C()« as D;
+
+    // $TEST$ no error "This type cast can never succeed."
+    »D()« as C;
+
+    // $TEST$ no error "This type cast can never succeed."
+    »unresolved« as Int;
 }