Default implicit argument gets mixed up with context bound

[travisbrown]

minimized code

trait Foo[A]; trait Bar

def qux1[A](implicit A: Foo[A], B: Bar = new Bar {}): Bar = B
def qux2[A: Foo](implicit B: Bar = new Bar {}): Bar = B

Compilation output

qux1 compiles as expected but qux2 does not:

1 |def qux2[A: Foo](implicit B: Bar = new Bar {}): Bar = B
  |                                             ^
  |                                           Found:    Object with Bar {...}
  |                                           Required: Foo[(?)]

expectation

It seems like both should compile, as they do on Scala 2. Note that the same thing happens if you replace implicit with using.