Fix mw.message plain() to substitute $N params in raw messages

src/wikitextprocessor/lua/mw_message.lua

@@ -127,7 +127,13 @@ local function makeMessage(options)
 
     function obj:plain()
         checkSelf(self, 'plain')
-        return data.keys
+        if not data.rawMessage then
+            return data.keys
+        end
+        -- https://www.mediawiki.org/wiki/Manual:Messages_API
+        return (data.rawMessage:gsub('$(%d+)', function(n)
+            return tostring(data.params[tonumber(n)] or '$' .. n)
+        end))
     end
 
     function obj:exists()

tests/test_lua.py

@@ -641,6 +641,36 @@ def test_el_strip_arg(self):
             "",
         )
 
+    def test_mw_message_raw_message_params(self):
+        # Module:Citation/CS1 substitutes message parameters with mw.message
+        self.wtp.start_page("")
+        self.wtp.add_page(
+            "Module:test",
+            828,
+            """local export = {}
+function export.test(frame)
+  return mw.message.newRawMessage("$1 and $2 and $3", "one", "two"):plain()
+end
+return export""",
+        )
+        self.assertEqual(
+            self.wtp.expand("{{#invoke:test|test}}"), "one and two and $3"
+        )
+
+    def test_mw_message_raw_message_double_digit_param(self):
+        self.wtp.start_page("")
+        self.wtp.add_page(
+            "Module:test",
+            828,
+            """local export = {}
+function export.test(frame)
+  return mw.message.newRawMessage("$10 $1", "one", "two", "three", "four",
+      "five", "six", "seven", "eight", "nine", "ten"):plain()
+end
+return export""",
+        )
+        self.assertEqual(self.wtp.expand("{{#invoke:test|test}}"), "ten one")
+
     def test_mw_site_canonical_ns_key(self):
         # https://cs.wiktionary.org/wiki/Modul:Maintenance#L-193
         # `mw.site.namespaces.Category.name`