Don't check bounds of Java applications in Java units

Something goes wrong when we try to do mutually recursive F-bounds checking in Java units.
I am not sure what exactly. But in any case, Scala should not try to do Java's typechecking.
So we now check applications in Java units only if they refer to Scala classes.

I added a test for scala#7494, which was originally addressed by scala#9370, the PR which introduced the Java bounds
checking. Adding the test ensures that the new restrictions still glag the original error.

Fixes scala#17763

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

@@ -145,7 +145,13 @@ object Checking {
     val checker = new TypeTraverser:
       def traverse(tp: Type) =
         tp match
-          case AppliedType(tycon, argTypes) =>
+          case AppliedType(tycon, argTypes)
+          if !(tycon.typeSymbol.is(JavaDefined) && ctx.compilationUnit.isJava) =>
+            // Don't check bounds in Java units that refer to Java type constructors.
+            // Scala is not obliged to do Java type checking and in fact i17763 goes wrong
+            // if we attempt to check bounds of F-bounded mutually recursive Java interfaces.
+            // Do check all bounds in Scala units and those bounds in Java units that
+            // occur in applications of Scala type constructors.
             checkAppliedType(
               untpd.AppliedTypeTree(TypeTree(tycon), argTypes.map(TypeTree(_)))
                 .withType(tp).withSpan(tpt.span.toSynthetic),

tests/neg/i7494/J.java

@@ -0,0 +1,7 @@
+public class J {
+  // for java, D is D<T extends Object>
+  public static D<String> getDS() { // error
+    return new D<String>(DS);
+  }
+}
+

tests/neg/i7494/S.scala

@@ -0,0 +1,10 @@
+class A
+
+class D[T >: A](v: T) {
+  def getV(): T = v // ensure T is correctly inferred
+}
+
+object S {
+  // J.getDS() : D[String] is inferred but not checked
+  val dv: String = J.getDS().getV()
+}

tests/pos/i17763/CopyableBuilder.java

@@ -0,0 +1,2 @@
+public interface CopyableBuilder<B extends CopyableBuilder<B, T>, T extends ToCopyableBuilder<B, T>> {
+}

tests/pos/i17763/Crash.scala

@@ -0,0 +1 @@
+class Crash

tests/pos/i17763/ToCopyableBuilder.java

@@ -0,0 +1,2 @@
+public interface ToCopyableBuilder<B extends CopyableBuilder<B, T>, T extends ToCopyableBuilder<B, T>> {
+}