When converting RGBA to PA, use RGB to P quantization

Author: radarhere

Tests/test_image_convert.py

@@ -107,6 +107,13 @@ 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_rgba() -> None:
     with Image.open("Tests/images/transparent.png") as im:
         assert im.mode == "RGBA"

src/PIL/Image.py

@@ -1010,8 +1010,14 @@ def convert_transparency(
                 new_im.info["transparency"] = transparency
             return new_im
 
-        if mode == "P" and self.mode == "RGBA":
-            return self.quantize(colors)
+        if self.mode == "RGBA":
+            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