scala/scala — pull request #3022 by retronym (2013-10-07 21:49:03+00:00)
Title: 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.

Review by @adriaanm.

- what about other uses of `inferExprInstance`
- can you suggest a test case to exercise `adjustTypeArgs(tparams, tvars, targsStrict)`, which isn't covered.