When converting RGBA to PA, use RGB to P quantization (#9153)

Tests/test_image.py

@@ -1099,6 +1099,12 @@ class TestImage:
             assert im.palette is not None
             assert im.palette.colors[(27, 35, 6, 214)] == 24
 
+    def test_merge_pa(self) -> None:
+        p = hopper("P")
+        a = Image.new("L", p.size)
+        pa = Image.merge("PA", (p, a))
+        assert p.getpalette() == pa.getpalette()
+
     def test_constants(self) -> None:
         for enum in (
             Image.Transpose,

Tests/test_image_convert.py

@@ -97,6 +97,13 @@ def test_opaque() -> None:
     assert_image_equal(alpha, solid)
 
 
+def test_rgba() -> None:
+    with Image.open("Tests/images/transparent.png") as im:
+        assert im.mode == "RGBA"
+
+        assert_image_similar(im.convert("RGBa").convert("RGB"), im.convert("RGB"), 1.5)
+
+
 def test_rgba_p() -> None:
     im = hopper("RGBA")
     im.putalpha(hopper("L"))
@@ -107,6 +114,13 @@ def test_rgba_p() -> None:
     assert_image_similar(im, comparable, 20)
 
 
+def test_rgba_pa() -> None:
+    im = hopper("RGBA").convert("PA").convert("RGB")
+    expected = hopper("RGB")
+
+    assert_image_similar(im, expected, 9.3)
+
+
 def test_pa() -> None:
     im = hopper().convert("PA")
 
@@ -115,13 +129,6 @@ def test_pa() -> None:
     assert palette.colors != {}
 
 
-def test_rgba() -> None:
-    with Image.open("Tests/images/transparent.png") as im:
-        assert im.mode == "RGBA"
-
-        assert_image_similar(im.convert("RGBa").convert("RGB"), im.convert("RGB"), 1.5)
-
-
 def test_trns_p(tmp_path: Path) -> None:
     im = hopper("P")
     im.info["transparency"] = 0

src/PIL/Image.py

@@ -1009,8 +1009,14 @@ class Image:
                 new_im.info["transparency"] = transparency
             return new_im
 
-        if mode == "P" and self.mode == "RGBA":
+        if self.mode == "RGBA":
-            return self.quantize(colors)
+            if mode == "P":
+                return self.quantize(colors)
+            elif mode == "PA":
+                r, g, b, a = self.split()
+                rgb = merge("RGB", (r, g, b))
+                p = rgb.quantize(colors)
+                return merge("PA", (p, a))
 
         trns = None
         delete_trns = False

src/_imaging.c

@@ -2419,7 +2419,12 @@ _merge(PyObject *self, PyObject *args) {
         bands[3] = band3->image;
     }
 
-    return PyImagingNew(ImagingMerge(mode, bands));
+    Imaging imOut = ImagingMerge(mode, bands);
+    if (!imOut) {
+        return NULL;
+    }
+    ImagingCopyPalette(imOut, bands[0]);
+    return PyImagingNew(imOut);
 }
 
 static PyObject *