Take expected result type into account more often for overloading resolution

Previously, the expected result type of a FunProto type was ignored and taken into
account only in case of ambiguities. arrayclone-new.scala shows that this is not enough.
In a case like

    val x: Array[Byte] = Array(1, 2)

we typed 1, 2 to be Int, so overloading resulution would give the Array.apply of
type (Int, Int*)Array[Int]. But that's a dead end, since Array[Int] is not a subtype
of Array[Byte].

This commit proposes the following modified rule for overloading resulution:

  A method alternative is applicable if ... (as before), and if its result type
  is copmpatible with the expected type of the method application.

The commit does not pre-select alternatives based on comparing with the expected
result type. I tried that but it slowed down typechecking by a factor of at least 4.
Instead, we proceed as usual, ignoring the result type except in case of
ambiguities, but check whether the result of overloading resolution has a
compatible result type. If that's not the case, we filter all alternatives
for result type compatibility and try again.

1 files changed, 109 insertions, 0 deletions

diff --git a/tests/run/arrayclone-new.scala b/tests/run/arrayclone-new.scala
new file mode 100644
index 000000000..8f66d1b31
--- /dev/null
+++ b/tests/run/arrayclone-new.scala
@@ -0,0 +1,109 @@
+import scala.reflect.ClassTag
+
+object Test extends dotty.runtime.LegacyApp{
+  BooleanArrayClone;
+  ByteArrayClone;
+  ShortArrayClone;
+  CharArrayClone;
+  IntArrayClone;
+  LongArrayClone;
+  FloatArrayClone;
+  DoubleArrayClone;
+  ObjectArrayClone;
+  PolymorphicArrayClone;
+}
+
+object BooleanArrayClone{
+  val it : Array[Boolean] = Array(true, false);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = false;
+  assert(it(0) == true)
+}
+
+object ByteArrayClone{
+  val it : Array[Byte] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object ShortArrayClone{
+  val it : Array[Short] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object CharArrayClone{
+  val it : Array[Char] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object IntArrayClone{
+  val it : Array[Int] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object LongArrayClone{
+  val it : Array[Long] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object FloatArrayClone{
+  val it : Array[Float] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object DoubleArrayClone{
+  val it : Array[Double] = Array(1, 0);
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = 0;
+  assert(it(0) == 1)
+}
+
+object ObjectArrayClone{
+  val it : Array[String] = Array("1", "0");
+  val cloned = it.clone();
+  assert(cloned.sameElements(it));
+  cloned(0) = "0";
+  assert(it(0) == "1")
+}
+
+object PolymorphicArrayClone{
+  def testIt[T](it : Array[T], one : T, zero : T) = {
+    val cloned = it.clone();
+    assert(cloned.sameElements(it));
+    cloned(0) = zero;
+    assert(it(0) == one)
+  }
+
+  testIt(Array("one", "two"), "one", "two");
+
+  class Mangler[T: ClassTag](ts : T*){
+    // this will always be a BoxedAnyArray even after we've unboxed its contents.
+    val it = ts.toArray[T];
+  }
+
+  val mangled = new Mangler[Int](0, 1);
+
+  val y : Array[Int] = mangled.it; // make sure it's unboxed
+
+  testIt(mangled.it, 0, 1);
+}
+