Don't check bounds of Java applications in Java units

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/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>> {
+}