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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.
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