#2541 fix incorrect name for TypeVar with ElementType.TYPE_USE for javac-with-errorprone

processor/src/main/java/org/mapstruct/ap/internal/model/common/TypeFactory.java

@@ -278,7 +278,17 @@ public class TypeFactory {
             isInterface = false;
             // When the component type is primitive and is annotated with ElementType.TYPE_USE then
             // the typeMirror#toString returns (@CustomAnnotation :: byte) for the javac compiler
-            name = mirror.getKind().isPrimitive() ? NativeTypes.getName( mirror.getKind() ) : mirror.toString();
+            if ( mirror.getKind().isPrimitive() ) {
+                name = NativeTypes.getName( mirror.getKind() );
+            }
+            // When the component type is type var and is annotated with ElementType.TYPE_USE then
+            // the typeMirror#toString returns (@CustomAnnotation T) for the errorprone javac compiler
+            else if ( mirror.getKind() == TypeKind.TYPEVAR ) {
+                name = ( (TypeVariable) mirror ).asElement().getSimpleName().toString();
+            }
+            else {
+                name = mirror.toString();
+            }
             packageName = null;
             qualifiedName = name;
             typeElement = null;

processor/src/test/java/org/mapstruct/ap/test/bugs/_2541/Issue2541Mapper.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.bugs._2541;
+
+import java.util.Optional;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+@Mapper
+public interface Issue2541Mapper {
+
+    Issue2541Mapper INSTANCE = Mappers.getMapper( Issue2541Mapper.class );
+
+    Target map(Source source);
+
+    default <T> Optional<T> toOptional(@Nullable T value) {
+        return Optional.ofNullable( value );
+    }
+
+    class Target {
+        private Optional<String> value;
+
+        public Optional<String> getValue() {
+            return value;
+        }
+
+        public void setValue(Optional<String> value) {
+            this.value = value;
+        }
+    }
+
+    class Source {
+        private final String value;
+
+        public Source(String value) {
+            this.value = value;
+        }
+
+        public String getValue() {
+            return value;
+        }
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2541/Issue2541Test.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.bugs._2541;
+
+import org.mapstruct.ap.testutil.ProcessorTest;
+import org.mapstruct.ap.testutil.WithClasses;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Filip Hrisafov
+ */
+@WithClasses({
+    Issue2541Mapper.class,
+    Nullable.class,
+})
+class Issue2541Test {
+
+    @ProcessorTest
+    void shouldGenerateCorrectCode() {
+        Issue2541Mapper.Target target = Issue2541Mapper.INSTANCE.map( new Issue2541Mapper.Source( null ) );
+
+        assertThat( target.getValue() ).isEmpty();
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2541/Nullable.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright MapStruct Authors.
+ *
+ * Licensed under the Apache License version 2.0, available at http://www.apache.org/licenses/LICENSE-2.0
+ */
+package org.mapstruct.ap.test.bugs._2541;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target({
+    ElementType.METHOD,
+    ElementType.TYPE_USE
+})
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+public @interface Nullable {
+
+}