From 39551242d7cfbab7e6bbadfed8c771b483cc1a13 Mon Sep 17 00:00:00 2001
From: Zegveld <41897697+Zegveld@users.noreply.github.com>
Date: Fri, 24 Jan 2025 14:41:35 +0100
Subject: [PATCH] #3786: Improve error message when mapping non-iterable to
 array

---
 .../processor/MethodRetrievalProcessor.java   |  5 +++
 .../mapstruct/ap/internal/util/Message.java   |  1 +
 .../bugs/_3786/ErroneousByteArrayMapper.java  | 13 +++++++
 .../ap/test/bugs/_3786/Issue3786Test.java     | 36 +++++++++++++++++++
 4 files changed, 55 insertions(+)
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/ErroneousByteArrayMapper.java
 create mode 100644 processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/Issue3786Test.java

diff --git a/processor/src/main/java/org/mapstruct/ap/internal/processor/MethodRetrievalProcessor.java b/processor/src/main/java/org/mapstruct/ap/internal/processor/MethodRetrievalProcessor.java
index 2c8ad5dcb..c8c13e6bf 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/processor/MethodRetrievalProcessor.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/processor/MethodRetrievalProcessor.java
@@ -555,6 +555,11 @@ public class MethodRetrievalProcessor implements ModelElementProcessor<Void, Lis
                 return false;
             }
 
+            if ( !parameterType.isIterableOrStreamType() && resultType.isArrayType() ) {
+                messager.printMessage( method, Message.RETRIEVAL_NON_ITERABLE_TO_ARRAY );
+                return false;
+            }
+
             if ( parameterType.isPrimitive() ) {
                 messager.printMessage( method, Message.RETRIEVAL_PRIMITIVE_PARAMETER );
                 return false;
diff --git a/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java b/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
index 5887ee07f..221aed285 100644
--- a/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
+++ b/processor/src/main/java/org/mapstruct/ap/internal/util/Message.java
@@ -175,6 +175,7 @@ public enum Message {
     RETRIEVAL_ITERABLE_TO_NON_ITERABLE( "Can't generate mapping method from iterable type from java stdlib to non-iterable type." ),
     RETRIEVAL_MAPPING_HAS_TARGET_TYPE_PARAMETER( "Can't generate mapping method that has a parameter annotated with @TargetType." ),
     RETRIEVAL_NON_ITERABLE_TO_ITERABLE( "Can't generate mapping method from non-iterable type to iterable type from java stdlib." ),
+    RETRIEVAL_NON_ITERABLE_TO_ARRAY( "Can't generate mapping method from non-iterable type to array." ),
     RETRIEVAL_PRIMITIVE_PARAMETER( "Can't generate mapping method with primitive parameter type." ),
     RETRIEVAL_PRIMITIVE_RETURN( "Can't generate mapping method with primitive return type." ),
     RETRIEVAL_TYPE_VAR_SOURCE( "Can't generate mapping method for a generic type variable source." ),
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/ErroneousByteArrayMapper.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/ErroneousByteArrayMapper.java
new file mode 100644
index 000000000..abf84f822
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/ErroneousByteArrayMapper.java
@@ -0,0 +1,13 @@
+/*
+ * 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._3786;
+
+import org.mapstruct.Mapper;
+
+@Mapper
+public interface ErroneousByteArrayMapper {
+    byte[] map( String something );
+}
diff --git a/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/Issue3786Test.java b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/Issue3786Test.java
new file mode 100644
index 000000000..bd71392c3
--- /dev/null
+++ b/processor/src/test/java/org/mapstruct/ap/test/bugs/_3786/Issue3786Test.java
@@ -0,0 +1,36 @@
+/*
+ * 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._3786;
+
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.ProcessorTest;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.compilation.annotation.CompilationResult;
+import org.mapstruct.ap.testutil.compilation.annotation.Diagnostic;
+import org.mapstruct.ap.testutil.compilation.annotation.ExpectedCompilationOutcome;
+
+/**
+ * @author Ben Zegveld
+ */
+@IssueKey( "3786" )
+public class Issue3786Test {
+
+    @WithClasses( ErroneousByteArrayMapper.class )
+    @ProcessorTest
+    @ExpectedCompilationOutcome(
+        value = CompilationResult.FAILED,
+        diagnostics = {
+            @Diagnostic(
+                type = ErroneousByteArrayMapper.class,
+                kind = javax.tools.Diagnostic.Kind.ERROR,
+                line = 12,
+                message = "Can't generate mapping method from non-iterable type to array."
+            )
+        }
+    )
+    void byteArrayReturnTypeShouldGiveInaccessibleContructorError() {
+    }
+}