Fail compilation of attempting a 0-arity product extraction

It's reasonable to desire this to work, but for now let's just not
crash.

Co-authored-by: Seth Tisue <seth@tisue.net>

Author: dwijnand

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -189,7 +189,7 @@ object Applications {
         getUnapplySelectors(getTp, args, pos)
       else if (unapplyResult.widenSingleton isRef defn.BooleanClass)
         Nil
-      else if (defn.isProductSubType(unapplyResult))
+      else if (defn.isProductSubType(unapplyResult) && productArity(unapplyResult, pos) != 0)
         productSelectorTypes(unapplyResult, pos)
           // this will cause a "wrong number of arguments in pattern" error later on,
           // which is better than the message in `fail`.

tests/neg/i13960.check

@@ -0,0 +1,6 @@
+-- [E108] Declaration Error: tests/neg/i13960.scala:13:10 --------------------------------------------------------------
+13 |    case A() => // error
+   |         ^^^
+   |          A is not a valid result type of an unapply method of an extractor.
+
+longer explanation available when compiling with `-explain`

tests/neg/i13960.scala

@@ -0,0 +1,15 @@
+class A() extends Product {
+  override def canEqual(that: Any) = true
+  override def productArity = 0
+  override def productElement(n: Int) = null
+}
+
+object A {
+  def unapply(a: A): A = a
+}
+
+object Main {
+  (new A) match {
+    case A() => // error
+  }
+}