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, 14 insertions, 0 deletions

diff --git a/test/files/run/t7584b.scala b/test/files/run/t7584b.scala
new file mode 100644
index 0000000000..fd560f0418
--- /dev/null
+++ b/test/files/run/t7584b.scala
@@ -0,0 +1,14 @@
+object Test extends App {
+  def fold[A, B](f: (A, => B) => B) = (b: B) => f(null.asInstanceOf[A], b)
+  def f[A, B](x: A, y: B): B = y
+  def bip[A, B] = fold[A, B]((x, y) => f(x, y))
+  def bop[A, B] = fold[A, B](f(_, _))
+
+  // these work:
+  fold[Int, Int]((x, y) => f(x, y))(0)
+  fold[Int, Int](f(_, _))(0)
+
+  // Used to throw a ClassCastException. Since the fix for SI-7899, these issue type errors.
+  // fold[Int, Int](f _)(0)
+  // fold[Int, Int](f)(0)
+}