#2233 Allow generic of generics in types when matching

e.g. method signature such as
<T> T fromOptional(Optional<T> optional)

when T resolves to another generic class such as Collection<String>

processor/src/main/java/org/mapstruct/ap/internal/model/source/MethodMatcher.java

@@ -296,8 +296,17 @@ public class MethodMatcher {
         public Boolean visitTypeVariable(TypeVariable t, TypeMirror p) {
             if ( genericTypesMap.containsKey( t ) ) {
                 // when already found, the same mapping should apply
+                // Then we should visit the resolved generic type.
+                // Which can potentially be another generic type
+                // e.g.
+                // <T> T fromOptional(Optional<T> optional)
+                // T resolves to Collection<Integer>
+                // We know what T resolves to, so we should treat it as if the method signature was
+                // Collection<Integer> fromOptional(Optional<Collection<Integer> optional)
                 TypeMirror p1 = genericTypesMap.get( t );
-                return typeUtils.isSubtype( p, p1 );
+                // p (Integer) should be a subType of p1 (Number)
+                // i.e. you can assign p (Integer) to p1 (Number)
+                return visit( p, p1 );
             }
             else {
                 // check if types are in bound

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/Issue2233Test.java

@@ -0,0 +1,50 @@
+/*
+ * 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._2233;
+
+import java.util.Collections;
+import java.util.Optional;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mapstruct.ap.testutil.IssueKey;
+import org.mapstruct.ap.testutil.WithClasses;
+import org.mapstruct.ap.testutil.runner.AnnotationProcessorTestRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.tuple;
+
+/**
+ * @author Filip Hrisafov
+ */
+@IssueKey("2233")
+@RunWith(AnnotationProcessorTestRunner.class)
+@WithClasses( {
+    Program.class,
+    ProgramAggregate.class,
+    ProgramDto.class,
+    ProgramMapper.class,
+    ProgramResponseDto.class,
+} )
+public class Issue2233Test {
+
+    @Test
+    public void shouldCorrectlyMapFromOptionalToCollection() {
+        ProgramResponseDto response = ProgramMapper.INSTANCE.map( new ProgramAggregate( Collections.singleton(
+            new Program(
+                "Optional Mapping",
+                "123"
+            ) ) ) );
+
+        assertThat( response ).isNotNull();
+        assertThat( response.getPrograms() ).isPresent();
+        assertThat( response.getPrograms().get() )
+            .extracting( ProgramDto::getName, ProgramDto::getNumber )
+            .containsExactly(
+                tuple( Optional.of( "Optional Mapping" ), Optional.of( "123" ) )
+            );
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/Program.java

@@ -0,0 +1,30 @@
+/*
+ * 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._2233;
+
+import java.util.Optional;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class Program {
+
+    private final String name;
+    private final String number;
+
+    public Program(String name, String number) {
+        this.name = name;
+        this.number = number;
+    }
+
+    public Optional<String> getName() {
+        return Optional.ofNullable( name );
+    }
+
+    public Optional<String> getNumber() {
+        return Optional.ofNullable( number );
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/ProgramAggregate.java

@@ -0,0 +1,25 @@
+/*
+ * 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._2233;
+
+import java.util.Collection;
+import java.util.Optional;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class ProgramAggregate {
+
+    private final Collection<Program> programs;
+
+    public ProgramAggregate(Collection<Program> programs) {
+        this.programs = programs;
+    }
+
+    public Optional<Collection<Program>> getPrograms() {
+        return Optional.ofNullable( programs );
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/ProgramDto.java

@@ -0,0 +1,30 @@
+/*
+ * 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._2233;
+
+import java.util.Optional;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class ProgramDto {
+
+    private final String name;
+    private final String number;
+
+    public ProgramDto(String name, String number) {
+        this.name = name;
+        this.number = number;
+    }
+
+    public Optional<String> getName() {
+        return Optional.ofNullable( name );
+    }
+
+    public Optional<String> getNumber() {
+        return Optional.ofNullable( number );
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/ProgramMapper.java

@@ -0,0 +1,31 @@
+/*
+ * 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._2233;
+
+import java.util.Collection;
+import java.util.Optional;
+
+import org.mapstruct.Mapper;
+import org.mapstruct.factory.Mappers;
+
+/**
+ * @author Filip Hrisafov
+ */
+@Mapper
+public interface ProgramMapper {
+
+    ProgramMapper INSTANCE = Mappers.getMapper( ProgramMapper.class );
+
+    ProgramResponseDto map(ProgramAggregate programAggregate);
+
+    ProgramDto map(Program sourceProgramDto);
+
+    Collection<ProgramDto> mapPrograms(Collection<Program> sourcePrograms);
+
+    default <T> T fromOptional(Optional<T> optional) {
+        return optional.orElse( null );
+    }
+}

processor/src/test/java/org/mapstruct/ap/test/bugs/_2233/ProgramResponseDto.java

@@ -0,0 +1,25 @@
+/*
+ * 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._2233;
+
+import java.util.Collection;
+import java.util.Optional;
+
+/**
+ * @author Filip Hrisafov
+ */
+public class ProgramResponseDto {
+
+    private final Collection<ProgramDto> programs;
+
+    public ProgramResponseDto(Collection<ProgramDto> programs) {
+        this.programs = programs;
+    }
+
+    public Optional<Collection<ProgramDto>> getPrograms() {
+        return Optional.ofNullable( programs );
+    }
+}