Fix #1747: Tweak for <:< between method types

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

@@ -487,7 +487,12 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case tp2 @ MethodType(_, formals2) =>
       def compareMethod = tp1 match {
         case tp1 @ MethodType(_, formals1) =>
-          (tp1.signature consistentParams tp2.signature) &&
+          val potentialMatch =
+            tp1.signature.consistentParams(tp2.signature) ||
+            tp1.isJava != tp2.isJava
+              // Overriding Java and non-Java methods might differ in their signature:
+              // Java maps Array[T] to Array[Object], Scala maps it to Object. See #1747.
+          potentialMatch &&
             matchingParams(formals1, formals2, tp1.isJava, tp2.isJava) &&
             (!tp1.isImplicit || tp2.isImplicit) &&  // non-implicit functions shadow implicit ones
             isSubType(tp1.resultType, tp2.resultType.subst(tp2, tp1))
@@ -1489,7 +1494,7 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
   }
 
   override def isSubType(tp1: Type, tp2: Type) =
-    traceIndented(s"${show(tp1)} <:< ${show(tp2)}${if (Config.verboseExplainSubtype) s" ${tp1.getClass} ${tp2.getClass}" else ""}${if (frozenConstraint) " frozen" else ""}") {
+    traceIndented(s"${show(tp1)} <:< ${show(tp2)}${if (Config.verboseExplainSubtype) s" ${tp1.getClass} ${tp2.getClass} ${tp1.signature} ${tp2.signature}" else ""}${if (frozenConstraint) " frozen" else ""}") {
       super.isSubType(tp1, tp2)
     }
 

tests/run/i1747-a.check

@@ -0,0 +1 @@
+523

tests/run/i1747-a/Foo.java

@@ -0,0 +1,4 @@
+
+public interface Foo<E> {
+    <T> T[] foo(T[] a, T e);
+}

tests/run/i1747-a/i1747.scala

@@ -0,0 +1,13 @@
+import java.util
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(new Bar[Int].foo(Array(1, 2, 3), 5).mkString)
+  }
+}
+
+class Bar[E] extends Foo[E] {
+  def foo[T](a: Array[T], e: T): Array[T] = {
+    a(0) = e
+    a
+  }
+}

tests/run/i1747-b.check

@@ -0,0 +1 @@
+null23

tests/run/i1747-b/i1747.scala

@@ -0,0 +1,27 @@
+import java.util
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(new Coll[Int].toArray(Array(1, 2, 3)).mkString)
+  }
+}
+
+class Coll[E] extends java.util.Collection[E] {
+
+  def toArray[T](a: Array[T]): Array[T] = {
+    a(0) = null.asInstanceOf[T]
+    a
+  }
+
+  def removeAll(c: util.Collection[_]): Boolean = ???
+  def retainAll(c: util.Collection[_]): Boolean = ???
+  def clear(): Unit = ???
+  def toArray: Array[AnyRef] = ???
+  def size(): Int = ???
+  def remove(o: scala.Any): Boolean = ???
+  def contains(o: scala.Any): Boolean = ???
+  def addAll(c: util.Collection[_ <: E]): Boolean = ???
+  def iterator(): util.Iterator[E] = ???
+  def isEmpty: Boolean = ???
+  def containsAll(c: util.Collection[_]): Boolean = ???
+  def add(e: E): Boolean = ???
+}