Add tests

Author: odersky

tests/neg/given-triangle.check

@@ -0,0 +1,4 @@
+-- [E172] Type Error: tests/neg/given-triangle.scala:14:18 -------------------------------------------------------------
+14 |@main def Test = f  // error
+   |                  ^
+   |Ambiguous given instances: both given instance given_B and given instance given_C match type A of parameter a of method f

tests/warn/given-triangle.scala renamed to tests/neg/given-triangle.scala

@@ -1,5 +1,3 @@
-//> using options -source 3.6-migration
-
 class A
 class B extends A
 class C extends A
@@ -13,4 +11,4 @@ def f(using a: A, b: B, c: C) =
   println(b.getClass)
   println(c.getClass)
 
-@main def Test = f  // warn
+@main def Test = f  // error

tests/warn/bson/Test.scala renamed to tests/pos/bson/Test.scala

@@ -0,0 +1,7 @@
+//> using options -Werror
+trait Writes[T]
+trait Format[T] extends Writes[T]
+given [T: List]: Writes[T] = null
+given [T]: Format[T] = null
+
+val _ = summon[Writes[Int]]

tests/warn/bson/bson.scala renamed to tests/pos/bson/bson.scala

@@ -0,0 +1,16 @@
+//> using options -source 3.5 -Werror
+trait SameRuntime[A, B]
+trait BSONWriter[T]
+trait BSONHandler[T] extends BSONWriter[T]
+
+opaque type Id = String
+object Id:
+  given SameRuntime[Id, String] = ???
+
+given BSONHandler[String]                    = ???
+given [T: BSONHandler]: BSONHandler[List[T]] = ???
+
+given opaqueWriter[T, A](using rs: SameRuntime[T, A], writer: BSONWriter[A]): BSONWriter[T] = ???
+
+val x = summon[BSONHandler[List[Id]]] // this doesn't emit warning
+val y = summon[BSONWriter[List[Id]]] // this did emit warning

tests/pos/i20572.scala

@@ -1,4 +1,4 @@
-import language.future
+import language.`3.6`
 
 class A
 class B extends A