fix type arg trailing comma parse error

Author: albertpchen

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1809,22 +1809,15 @@ object Parsers {
         NamedArg(name.toTypeName, argType())
       }
 
-      def otherArgs(first: Tree, arg: () => Tree): List[Tree] = {
-        val rest =
-          if (in.token == COMMA) {
-            in.nextToken()
-            commaSeparated(arg)
-          }
-          else Nil
-        first :: rest
-      }
       if (namedOK && in.token == IDENTIFIER)
-        argType() match {
-          case Ident(name) if in.token == EQUALS =>
-            in.nextToken()
-            otherArgs(NamedArg(name, argType()), () => namedTypeArg())
-          case firstArg =>
-            otherArgs(firstArg, () => argType())
+        in.currentRegion.withCommasExpected {
+          argType() match {
+            case Ident(name) if in.token == EQUALS =>
+              in.nextToken()
+              commaSeparatedRest(NamedArg(name, argType()), () => namedTypeArg())
+            case firstArg =>
+              commaSeparatedRest(firstArg, () => argType())
+          }
         }
       else commaSeparated(() => argType())
     }

tests/pos/trailingCommas/trailingCommas.scala

@@ -85,6 +85,19 @@ trait FunTypeParamClause {
   A,
   B,
   ]: Unit
+
+  def f1[
+  A,
+  ]: Unit
+
+  def g: Unit = f[
+    Int,
+    String,
+  ]
+
+  def g1: Unit = f1[
+    Int,
+  ]
 }
 
 trait SimpleType {