Fix exhaustivity warning (#16168)

Author: KacperFKorban

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -875,6 +875,10 @@ object TypeOps:
     }
 
     def instantiate(): Type = {
+      // if there's a change in variance in type parameters (between subtype tp1 and supertype tp2)
+      // then we don't want to maximise the type variables in the wrong direction.
+      // For instance 15967, A[-Z] and B[Y] extends A[Y], we don't want to maximise Y to Any
+      maximizeType(protoTp1.baseType(tp2.classSymbol), NoSpan)
       maximizeType(protoTp1, NoSpan)
       wildApprox(protoTp1)
     }

tests/pos/i15967.scala

@@ -0,0 +1,10 @@
+// scalac: -Werror
+sealed trait A[-Z]
+final case class B[Y]() extends A[Y]
+
+class Test:
+  def t1[X](a: A[X]) = a match // was inexhaustive
+    case _: B[X] @unchecked =>
+
+//def t2[X](a: A[X]) = a match // was inexhaustive
+//  case _: B[X] => // expected unchecked warning