SI-7899 Don't infer by-name types during, e.g. eta-expansion

Given:

    def id[A](a: A): A = a
    def foo(f: (=> Int) => Int) = ()
    foo(id)

We eta-expanded `id` and inferred `A` to be `=> Int` to satisfy the
expected type set forth by the formal parameter `f`.

We really shouldn't go about inferring types that we can't *write*.
Our attempt to do so led promptly into a `ClassCastException` in the
enclosed test.

This commit:

  - drops by-name-ness during `inferExprInstance`
  - tests that this results in a type error for the reported bug
    (neg/t7899)
  - tests that a method with a by-name parameter can still be
    eta expanded to match function with a corresponding by-name
    parameter (run/t7899)
  - discovers the same latent CCE in pos/t7584
     - now that would be a type error
     - so we compensate by using placeholder functions rather than
       eta-expansion.
     - and move that that test to `run` for good measure.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/files/neg/t7899.scala b/test/files/neg/t7899.scala
new file mode 100644
index 0000000000..f2dea3ab1f
--- /dev/null
+++ b/test/files/neg/t7899.scala
@@ -0,0 +1,7 @@
+object Test {
+  def foo[B](f: (=> Int) => B): () => B = () => f(0)
+
+  def main(args: Array[String]) {
+    foo(identity)()
+  }
+}