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

@@ -653,6 +653,10 @@ public D convert(S value) {
 		if (converter != null) {
 			try {
 				return converter.convert(value);
+			} catch (ClassCastException ex) {
+				String msg = BindingMessages.getString(BindingMessages.TYPE_ERASURE);
+				Policy.getLog().log(new Status(IStatus.ERROR, Activator.PLUGIN_ID, msg, ex));
+				throw ex;
 			} catch (Exception ex) {
 				Policy.getLog().log(new Status(IStatus.ERROR, Activator.PLUGIN_ID, ex.getMessage(), ex));
 				throw ex;
@@ -677,6 +681,10 @@ static final class DefaultConverter implements IConverter<Object, Object> {
 
 		@Override
 		public Object convert(Object fromObject) {
+			// Explicit cast necessary due to potential type erasure
+			if (toType instanceof Class<?> clazz) {
+				return clazz.cast(fromObject);
+			}
 			return fromObject;
 		}
 

bundles/org.eclipse.core.databinding/src/org/eclipse/core/internal/databinding/BindingMessages.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2005, 2008 IBM Corporation and others.
+ * Copyright (c) 2005, 2025 IBM Corporation and others.
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -110,6 +110,11 @@ public class BindingMessages {
 	 */
 	public static final String VALIDATE_NUMBER_PARSE_ERROR_NO_CHARACTER = "Validate_NumberParseErrorNoCharacter"; //$NON-NLS-1$
 
+	/**
+	 * Key to be used for a "TYPE_ERASURE" message
+	 */
+	public static final String TYPE_ERASURE = "Type_Erasure"; //$NON-NLS-1$
+
 	/**
 	 * Returns the resource object with the given key in the resource bundle for
 	 * JFace Data Binding. If there isn't any value under the given key, the key

bundles/org.eclipse.core.databinding/src/org/eclipse/core/internal/databinding/messages.properties

@@ -1,5 +1,5 @@
 ###############################################################################
-# Copyright (c) 2000, 2008 IBM Corporation and others.
+# Copyright (c) 2000, 2025 IBM Corporation and others.
 #
 # This program and the accompanying materials
 # are made available under the terms of the Eclipse Public License 2.0
@@ -56,4 +56,6 @@ Validate_ConversionFromClassToPrimitive="Wrong object type to convert to primiti
 Validate_NoChangeAllowedHelp=Changes are not allowed in this field
 Validate_CharacterHelp=Please type a character
 
-Examples=Examples
+Examples=Examples
+
+Type_Erasure=Value was not converted and type erasure occurred.

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);
+	}
 }