Improved message for missing argument list

Clarifies the language and rules for eta-expansion.

A missing argument in a list, as opposed to a missing
argument list, results in a different message.

The comical expansion in parens does not attempt to show
what was already applied, but succeeds in showing at a
glance the shape of the method in question.

```
scala> def m(i: Int, j: Int)(x: Int) = ???
m: (i: Int, j: Int)(x: Int)Nothing

scala> m
<console>:12: error: missing argument list for method m
Unapplied methods are only converted to functions when a function type is expected.
You can make this conversion explicit by writing `m _` or `m(_,_)(_)` instead of `m`.
       m
       ^
```

The original submission was due to sschaef and the wording due
to adriaanm, with a minor tweak.

1 files changed, 21 insertions, 0 deletions

diff --git a/test/files/neg/missing-arg-list.check b/test/files/neg/missing-arg-list.check
new file mode 100644
index 0000000000..5a011c36f2
--- /dev/null
+++ b/test/files/neg/missing-arg-list.check
@@ -0,0 +1,21 @@
+missing-arg-list.scala:9: error: missing argument list for method id in trait T
+Unapplied methods are only converted to functions when a function type is expected.
+You can make this conversion explicit by writing `id _` or `id(_)` instead of `id`.
+  val w = id
+          ^
+missing-arg-list.scala:10: error: missing argument list for method f in trait T
+Unapplied methods are only converted to functions when a function type is expected.
+You can make this conversion explicit by writing `f _` or `f(_)(_)` instead of `f`.
+  val x = f
+          ^
+missing-arg-list.scala:11: error: missing argument list for method g in trait T
+Unapplied methods are only converted to functions when a function type is expected.
+You can make this conversion explicit by writing `g _` or `g(_,_,_)` instead of `g`.
+  val y = g
+          ^
+missing-arg-list.scala:12: error: missing argument list for method h in trait T
+Unapplied methods are only converted to functions when a function type is expected.
+You can make this conversion explicit by writing `h _` or `h(_,_,_)(_)` instead of `h`.
+  val z = h
+          ^
+four errors found