Constant fold results of nonempty tests in pattern matching

Fixes #11605

Author: odersky

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -692,9 +692,10 @@ object PatternMatcher {
       val scrutinee = plan.scrutinee
       (plan.test: @unchecked) match {
         case NonEmptyTest =>
-          scrutinee
-            .select(nme.isEmpty, _.info.isParameterless)
-            .select(nme.UNARY_!, _.info.isParameterless)
+          constToLiteral(
+            scrutinee
+              .select(nme.isEmpty, _.info.isParameterless)
+              .select(nme.UNARY_!, _.info.isParameterless))
         case NonNullTest =>
           scrutinee.testNotNull
         case GuardTest =>

tests/pos/i11605.scala

@@ -0,0 +1,14 @@
+package example
+
+class Extractor[A](val get: A) extends AnyVal {
+  // https://github.com/scala/scala/pull/9343
+  def isEmpty: false = false
+}
+
+object Foo {
+  def unapply(x: String): Extractor[String] = new Extractor(x)
+}
+
+object Main {
+  val Foo(a) = "a"
+}