Fix wxFileName::MakeRelativeTo() for directory relatively to itself.

The expected result in this case is ".", but the filename became empty instead
when wxPATH_NATIVE was used.

Fix this by examining GetFormat(format), which takes care of mapping
wxPATH_NATIVE to its real value, instead of wxPATH_NATIVE itself.

Also add a unit test verifying that this works as expected.

Closes #17010.

docs/changes.txt

@@ -577,6 +577,10 @@ Major new features in this release
 3.0.3: (released 2015-??-??)
 ----------------------------
 
+All:
+
+- Fix wxFileName::MakeRelativeTo() for directory relatively to itself.
+
 Unix:
 
 - Fix wxIPaddress::Hostname() failing if /etc/hosts contained very long names.

src/common/filename.cpp

@@ -1832,15 +1832,27 @@ bool wxFileName::MakeRelativeTo(const wxString& pathBase, wxPathFormat format)
         m_dirs.Insert(wxT(".."), 0u);
     }
 
-    if ( format == wxPATH_UNIX || format == wxPATH_DOS )
+    switch ( GetFormat(format) )
     {
-        // a directory made relative with respect to itself is '.' under Unix
+        case wxPATH_NATIVE:
-        // and DOS, by definition (but we don't have to insert "./" for the
+        case wxPATH_MAX:
-        // files)
+            wxFAIL_MSG( wxS("unreachable") );
-        if ( m_dirs.IsEmpty() && IsDir() )
+            wxFALLTHROUGH;
-        {
+
-            m_dirs.Add(wxT('.'));
+        case wxPATH_UNIX:
-        }
+        case wxPATH_DOS:
+            // a directory made relative with respect to itself is '.' under
+            // Unix and DOS, by definition (but we don't have to insert "./"
+            // for the files)
+            if ( m_dirs.IsEmpty() && IsDir() )
+            {
+                m_dirs.Add(wxT('.'));
+            }
+            break;
+
+        case wxPATH_MAC:
+        case wxPATH_VMS:
+            break;
     }
 
     m_relative = true;

tests/filename/filenametest.cpp

@@ -133,6 +133,7 @@ private:
         CPPUNIT_TEST( TestSetPath );
         CPPUNIT_TEST( TestStrip );
         CPPUNIT_TEST( TestNormalize );
+        CPPUNIT_TEST( TestRelative );
         CPPUNIT_TEST( TestReplace );
         CPPUNIT_TEST( TestGetHumanReadable );
 #ifdef __WINDOWS__
@@ -158,6 +159,7 @@ private:
     void TestSplit();
     void TestSetPath();
     void TestStrip();
+    void TestRelative();
     void TestNormalize();
     void TestReplace();
     void TestGetHumanReadable();
@@ -441,6 +443,21 @@ void FileNameTestCase::TestNormalize()
 #endif // __WINDOWS__
 }
 
+void FileNameTestCase::TestRelative()
+{
+    wxFileName fn("a/b.cpp");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "b.cpp", fn.GetFullPath() );
+
+    fn.AssignDir("a/b");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "b/", fn.GetFullPath() );
+
+    fn.AssignDir("a");
+    fn.MakeRelativeTo("a");
+    CPPUNIT_ASSERT_EQUAL( "./", fn.GetFullPath() );
+}
+
 void FileNameTestCase::TestReplace()
 {
     static const struct FileNameTest