Add a minimisation for joining OrTypes in TupleN test

Author: dwijnand

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

@@ -336,7 +336,8 @@ object PatternMatcher {
 
         if (isSyntheticScala2Unapply(unapp.symbol) && caseAccessors.length == args.length)
           def tupleSel(sym: Symbol) = ref(scrutinee).select(sym)
-          val isGenericTuple = defn.isTupleClass(caseClass) && !defn.isTupleNType(tree.tpe)
+          val isGenericTuple = defn.isTupleClass(caseClass) &&
+            !defn.isTupleNType(tree.tpe match { case tp: OrType => tp.join case tp => tp }) // widen even hard unions, to see if it's a union of tuples
           val components = if isGenericTuple then caseAccessors.indices.toList.map(tupleApp(_, ref(scrutinee))) else caseAccessors.map(tupleSel)
           matchArgsPlan(components, args, onSuccess)
         else if (unapp.tpe <:< (defn.BooleanType))

tests/pos/i14587.hard-union-tuples.scala

@@ -0,0 +1,10 @@
+// scalac: -Werror
+class Test:
+  type Foo = Option[String] | Option[Int]
+
+  def test(foo: Foo) =
+    val (_, foo2: Foo) = // was: the type test for Test.this.Foo cannot be checked at runtime
+      foo match
+        case Some(s: String) => ((), s)
+        case _               => ((), 0)
+    foo2