SI-8129 Make Object#== override Any#==

And the same for !=

If we tried to declare these signatures in non-fictional classes,
we would be chastised about collapsing into the "same signature after
erasure".

This will have an influence of typing, as the typechecking of
arguments is sensitive to overloading: if multiple variants are
feasible, the argument will be typechecked with a wildcard expected
type. So people inspecting the types of the arguments to `==` before
this change might have seen an interesting type for
`if (true) x else y`, but now the `If` will have type `Any`, as we
don't need to calculate the LUB.

I've left a TODO to note that we should really make `Any#{==, !=}`
non-final and include a final override in `AnyVal`. But I don't think
that is particularly urgent.

1 files changed, 15 insertions, 0 deletions

diff --git a/test/files/pos/t8219.scala b/test/files/pos/t8219.scala
new file mode 100644
index 0000000000..e1653b6238
--- /dev/null
+++ b/test/files/pos/t8219.scala
@@ -0,0 +1,15 @@
+trait Equalizer[T]
+trait Gen[A]
+
+class Broken {
+  implicit def const[T](x: T): Gen[T] = ???
+  implicit def convertToEqualizer[T](left: T): Equalizer[T] = ???
+
+  def in(a: Any) = ()
+  in {
+    import scala.None // any import will do..
+    "" == "" // this no longer triggers the bug, as Object#== now overrides Any#==
+  }
+
+  // We can still trigger the bug with a structural type, see pending/neg/t8219.scala
+}