[MNG-6723] - Reproducer test

maven-core/src/main/java/org/apache/maven/project/DefaultProjectBuilder.java

@@ -976,6 +976,10 @@ private void initParent( MavenProject project, Map<String, MavenProject> project
                 }
             }
             project.setParent( parent );
+            if ( project.getParentFile() == null && parent != null )
+            {
+                project.setParentFile( parent.getFile() );
+            }
         }
     }
 

maven-core/src/test/java/org/apache/maven/project/ProjectBuilderTest.java

@@ -20,6 +20,7 @@
  */
 
 import java.io.File;
+import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Properties;
@@ -245,4 +246,57 @@ public void testReadInvalidPom()
         }
     }
 
+    public void testReadParentAndChildWithRegularVersionSetParentFile()
+            throws Exception
+    {
+        List<File> toRead = new ArrayList<>( 2 );
+        File parentPom = getProject( "MNG-6723" );
+        toRead.add( parentPom );
+        toRead.add( new File( parentPom.getParentFile(), "child/pom.xml" ) );
+        MavenSession mavenSession = createMavenSession( null );
+        ProjectBuildingRequest configuration = new DefaultProjectBuildingRequest();
+        configuration.setValidationLevel( ModelBuildingRequest.VALIDATION_LEVEL_MINIMAL );
+        configuration.setRepositorySession( mavenSession.getRepositorySession() );
+        org.apache.maven.project.ProjectBuilder projectBuilder =
+            lookup( org.apache.maven.project.ProjectBuilder.class );
+
+        // read poms separately
+        boolean parentFileWasFoundOnChild = false;
+        for ( File file : toRead ) {
+            List<ProjectBuildingResult> results = projectBuilder.build( Collections.singletonList( file ), false, configuration );
+            assertResultShowNoError( results );
+            MavenProject project = findChildProject( results );
+            if ( project != null ) {
+                assertEquals( parentPom, project.getParentFile() );
+                parentFileWasFoundOnChild = true;
+            }
+        }
+        assertTrue( parentFileWasFoundOnChild );
+
+        // read projects together
+        List<ProjectBuildingResult> results = projectBuilder.build( toRead, false, configuration );
+        assertResultShowNoError( results );
+        assertEquals( parentPom , findChildProject( results ).getParentFile() );
+        Collections.reverse( toRead );
+        results = projectBuilder.build( toRead, false, configuration );
+        assertResultShowNoError( results );
+        assertEquals( parentPom , findChildProject( results ).getParentFile() );
+    }
+
+    private MavenProject findChildProject(List<ProjectBuildingResult> results) {
+        for ( ProjectBuildingResult result : results ) {
+            if ( result.getPomFile().getParentFile().getName().equals( "child" ) ) {
+                return result.getProject();
+            }
+        }
+        return null;
+    }
+
+	private void assertResultShowNoError(List<ProjectBuildingResult> results) {
+        for ( ProjectBuildingResult result : results ) {
+            assertTrue( result.getProblems().isEmpty() );
+            assertNotNull( result.getProject() );
+        }
+    }
+
 }

maven-core/src/test/projects/project-builder/MNG-6723/child/pom.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<parent>
+		<groupId>example.eclipse-548652</groupId>
+		<artifactId>parent</artifactId>
+		<version>0.0.1-SNAPSHOT</version>
+	</parent>
+	<artifactId>child</artifactId>
+	<packaging>jar</packaging>
+</project>

maven-core/src/test/projects/project-builder/MNG-6723/pom.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>example.eclipse-548652</groupId>
+	<artifactId>parent</artifactId>
+	<version>0.0.1-SNAPSHOT</version>
+	<packaging>pom</packaging>
+	<modules>
+		<module>child</module>
+	</modules>
+</project>