Add explicit type check to UpdateStrategy.convert()

When using the DefaultConverter, type erasure may occur, where the
"convert" method returns an object of type S, but treats it as type D.

The conversion must fail early in such a case, in order to prevent
obscure error messages when passing the value to successive methods.

Closes #3008

Author: ptziegler

bundles/org.eclipse.core.databinding/src/org/eclipse/core/databinding/UpdateStrategy.java

@@ -652,7 +652,12 @@ protected IStatus logErrorWhileSettingValue(Exception ex) {
 	public D convert(S value) {
 		if (converter != null) {
 			try {
-				return converter.convert(value);
+				// Explicit cast necessary due to potential type erasure
+				D result = converter.convert(value);
+				if (converter.getToType() instanceof Class<?> clazz) {
+					return (D) clazz.cast(result);
+				}
+				return result;
 			} catch (Exception ex) {
 				Policy.getLog().log(new Status(IStatus.ERROR, Activator.PLUGIN_ID, ex.getMessage(), ex));
 				throw ex;

tests/org.eclipse.jface.tests.databinding/src/org/eclipse/core/tests/databinding/UpdateStrategyTest.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2007 IBM Corporation and others.
+ * Copyright (c) 2007, 2025 IBM Corporation and others.
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -16,11 +16,15 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.util.Date;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
 
 import org.eclipse.core.databinding.UpdateValueStrategy;
 import org.eclipse.core.databinding.conversion.IConverter;
@@ -354,4 +358,16 @@ public UpdateValueStrategy<S, D> setConverter(IConverter<? super S, ? extends D>
 			return super.setConverter(converter);
 		}
 	}
+
+	@Test
+	public void testDefaultConverterWithTypeErasure() {
+		WritableValue<Set<?>> source = WritableValue.withValueType(Set.class);
+		WritableValue<List<?>> destination = WritableValue.withValueType(List.class);
+
+		UpdateStrategyStub<Set<?>, List<?>> strategy = new UpdateStrategyStub<>();
+		strategy.fillDefaults(source, destination);
+
+		RuntimeException ex = assertThrows(RuntimeException.class, () -> strategy.convert(new HashSet<>()));
+		assertTrue("Type erasure was missed", ex.getCause() instanceof ClassCastException);
+	}
 }