Test remap_palette()'s source_palette argument

Tests/test_image.py

@@ -674,6 +674,23 @@ def test_remap_palette(self) -> None:
             with pytest.raises(ValueError):
                 im_hopper.remap_palette([])
 
+    @pytest.mark.parametrize("palette_mode", ("RGB", "RGBA"))
+    def test_remap_palette_source_palette(self, palette_mode: str) -> None:
+        # When source_palette is provided, mode is inferred from length.
+        # 768 entries should be detected as a 256-entry RGB palette
+        # 1024 entries should be detected as a 256-entry RGBA palette
+        im = Image.new("P", (1, 1))
+        source_palette = bytes(
+            entry for entry in range(256) for channel in range(len(palette_mode))
+        )
+        im_remapped = im.remap_palette(list(range(256)), source_palette)
+
+        palette = im_remapped.palette
+        assert palette is not None
+        assert len(palette.colors) == 256
+        assert palette.mode == palette_mode
+        assert palette.palette == source_palette
+
     def test_remap_palette_transparency(self) -> None:
         im = Image.new("P", (1, 2), (0, 0, 0))
         im.putpixel((0, 1), (255, 0, 0))