Use correct type for deriveFromProperties

Fix `DataSourceBuilder` so that the type used to access `deriveFrom`
properties is based on the actual instance type rather than the
user-defined type which could have been changed.

Fixes gh-26644

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jdbc/DataSourceBuilder.java

@@ -91,7 +91,7 @@ public final class DataSourceBuilder<T extends DataSource> {
 
 	private Class<T> type;
 
-	private final T deriveFrom;
+	private final DataSource deriveFrom;
 
 	private DataSourceBuilder(ClassLoader classLoader) {
 		this.classLoader = classLoader;
@@ -168,27 +168,20 @@ public final class DataSourceBuilder<T extends DataSource> {
 	 */
 	public T build() {
 		DataSourceProperties<T> properties = DataSourceProperties.forType(this.classLoader, this.type);
-		DataSourceProperties<T> deriveFromProperties = (this.deriveFrom != null)
-				? DataSourceProperties.forType(this.classLoader, this.type) : null;
+		DataSourceProperties<DataSource> deriveFromProperties = getDeriveFromProperties();
 		Class<? extends T> instanceType = (this.type != null) ? this.type : properties.getDataSourceInstanceType();
 		T dataSource = BeanUtils.instantiateClass(instanceType);
 		Set<DataSourceProperty> applied = new HashSet<>();
 		for (DataSourceProperty property : DataSourceProperty.values()) {
-			if (this.values.containsKey(property)) {
 			String value = this.values.get(property);
-				if (value != null) {
-					properties.set(dataSource, property, value);
-					applied.add(property);
+			if (!this.values.containsKey(property) && deriveFromProperties != null && properties.canSet(property)) {
+				value = deriveFromProperties.get(this.deriveFrom, property);
 			}
-			}
-			else if (deriveFromProperties != null && properties.canSet(property)) {
-				String value = deriveFromProperties.get(this.deriveFrom, property);
 			if (value != null) {
 				properties.set(dataSource, property, value);
 				applied.add(property);
 			}
 		}
-		}
 		if (!applied.contains(DataSourceProperty.DRIVER_CLASS_NAME)
 				&& properties.canSet(DataSourceProperty.DRIVER_CLASS_NAME)
 				&& this.values.containsKey(DataSourceProperty.URL)) {
@@ -199,6 +192,14 @@ public final class DataSourceBuilder<T extends DataSource> {
 		return dataSource;
 	}
 
+	@SuppressWarnings("unchecked")
+	private DataSourceProperties<DataSource> getDeriveFromProperties() {
+		if (this.deriveFrom == null) {
+			return null;
+		}
+		return DataSourceProperties.forType(this.classLoader, (Class<DataSource>) this.deriveFrom.getClass());
+	}
+
 	/**
 	 * Create a new {@link DataSourceBuilder} instance.
 	 * @return a new datasource builder instance

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/jdbc/DataSourceBuilderTests.java

@@ -318,6 +318,19 @@ class DataSourceBuilderTests {
 		assertThat(built.getUrl()).startsWith("jdbc:hsqldb:mem");
 	}
 
+	@Test // gh-26644
+	void buildWhenDerivedFromExistingDatabaseWithTypeChange() {
+		HikariDataSource dataSource = new HikariDataSource();
+		dataSource.setUsername("test");
+		dataSource.setPassword("secret");
+		dataSource.setJdbcUrl("jdbc:postgresql://localhost:5432/postgres");
+		DataSourceBuilder<?> builder = DataSourceBuilder.derivedFrom(dataSource).type(SimpleDriverDataSource.class);
+		SimpleDriverDataSource built = (SimpleDriverDataSource) builder.username("test2").password("secret2").build();
+		assertThat(built.getUsername()).isEqualTo("test2");
+		assertThat(built.getPassword()).isEqualTo("secret2");
+		assertThat(built.getUrl()).isEqualTo("jdbc:postgresql://localhost:5432/postgres");
+	}
+
 	final class HidePackagesClassLoader extends URLClassLoader {
 
 		private final String[] hiddenPackages;