test: add test for #19762

Add a test to confirm issue#19762 is solved.

[Cherry-picked 2a1d1b3]

Author: i10416

tests/neg/i19762.check

@@ -0,0 +1,6 @@
+-- [E019] Syntax Error: tests/neg/i19762.scala:8:31 --------------------------------------------------------------------
+8 |    def combine(x: Int, y: Int)) = x + y // error
+  |                               ^
+  |                               Missing return type
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i19762.scala

@@ -0,0 +1,8 @@
+trait Monoid[A]:
+  def combine(x: A, y: A): A
+  def empty: A
+
+object Monoid:
+  lazy val addInt: Monoid[Int] = new:
+    val empty = 0
+    def combine(x: Int, y: Int)) = x + y // error