Add tests for trailing comma parsing

tests/neg/t11900.check

@@ -0,0 +1,18 @@
+-- Error: tests/neg/t11900.scala:44:16 ---------------------------------------------------------------------------------
+44 |      a => a + 1,   // error: weird comma
+   |                ^
+   |                end of statement expected but ',' found
+-- Error: tests/neg/t11900.scala:48:16 ---------------------------------------------------------------------------------
+48 |    println("a"),   // error: weird comma
+   |                ^
+   |                end of statement expected but ',' found
+-- Error: tests/neg/t11900.scala:52:16 ---------------------------------------------------------------------------------
+52 |    println("b"),   // error: weird comma
+   |                ^
+   |                end of statement expected but ',' found
+-- [E032] Syntax Error: tests/neg/t11900.scala:64:8 --------------------------------------------------------------------
+64 |      _*, // error
+   |        ^
+   |        pattern expected
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/t11900.scala

@@ -0,0 +1,79 @@
+
+trait t11900 {
+  // cf pos/trailing-commas
+  //
+  import scala.collection.{
+    immutable,
+    mutable,
+  }
+
+  def h[A,
+  ]: List[A] = Nil
+
+  def u(
+      x: Int,
+      y: Int,
+  )(using List[Int],
+      Set[Int],
+  )(using l: List[Int],
+    s : Set[Int],
+  ): Int = 1
+
+  def g = List(
+    1,
+    2,
+    3,
+  )
+
+  def star =
+    List(1, 2, 3, 4, 5) match {
+      case List(
+      1,
+      2,
+      3,
+      ) => false
+      case List(
+      1,
+      2,
+      _*,
+      ) => true
+    }
+
+  def f =
+    List(1, 2, 3).map {
+      a => a + 1,   // error: weird comma
+    }
+
+  class A() {
+    println("a"),   // error: weird comma
+  }
+
+  def b() = {
+    println("b"),   // error: weird comma
+  }
+
+  def starcrossed =
+    List(1, 2, 3, 4, 5) match {
+      case List(
+      1,
+      2,
+      3,
+      ) => false
+      case List(
+      1,
+      _*, // error
+      2,
+      ) => true
+    }
+
+  def p(p: (Int,
+      String,
+      )
+  ): Unit
+
+  def q: (Int,
+      String,
+      )
+
+  val z = 42
+}

tests/neg/trailingCommas.scala

@@ -56,3 +56,27 @@ object `package` {
   case class Foo(foo: Any)
   case class Bar(foo: Any)
 }
+
+// Unparenthesized lists
+trait Deriv1[T]
+object Deriv1 {
+  def derived[T]: Deriv1[T] = new Deriv1[T] {}
+}
+
+trait Deriv2[T]
+object Deriv2 {
+  def derived[T]: Deriv2[T] = new Deriv2[T] {}
+}
+
+class Derives1 derives Deriv1, Deriv2,
+object End // error: an identifier expected, but 'object' found
+
+class Derives2 derives Deriv1,
+     Deriv2,
+object End2 // error: an identifier expected, but 'object' found
+
+val a,
+    b,
+    c,
+    = (1, 2, 3) // error
+val x, y, z, = (1, 2, 3) // error

tests/pos/comma-separated.scala

@@ -0,0 +1,19 @@
+trait Bar[T]
+object Bar {
+  def derived[T]: Bar[T] = new Bar[T] {}
+}
+
+trait Baz[T]
+object Baz {
+  def derived[T]: Baz[T] = new Baz[T] {}
+}
+
+class Foo derives Bar, Baz
+
+class Foo2 derives Bar,
+   Baz
+
+val x, y, z = (1, 2, 3)
+val a,
+    b,
+    c = (1, 2, 3)