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, 8 insertions, 0 deletions

diff --git a/test/files/neg/t8219-any-any-ref-equals.scala b/test/files/neg/t8219-any-any-ref-equals.scala
new file mode 100644
index 0000000000..f1b81fa734
--- /dev/null
+++ b/test/files/neg/t8219-any-any-ref-equals.scala
@@ -0,0 +1,8 @@
+object Test {
+  // The error message tells us that AnyRef#== and Any#== are overloaded.
+  // A real class couldn't define such an overload, why do we allow AnyRef
+  // to do so?
+  "".==[Int]
+  ("": AnyRef).==[Int]
+  ("": Object).==[Int]
+}