Propagate overloaded function type to expected arg type

Infer missing parameter types for function literals passed
to higher-order overloaded methods by deriving the
expected argument type from the function types in the
overloaded method type's argument types.

This eases the pain caused by methods becoming overloaded
because SAM types and function types are compatible,
which used to disable parameter type inference because
for overload resolution arguments are typed without
expected type, while typedFunction needs the expected
type to infer missing parameter types for function literals.

It also aligns us with dotty. The special case for
function literals seems reasonable, as it has precedent,
and it just enables the special case in typing function
literals (derive the param types from the expected type).

Since this does change type inference, you can opt out
using the Scala 2.11 source level.

Fix scala/scala-dev#157

1 files changed, 7 insertions, 7 deletions

diff --git a/test/files/neg/sammy_overload.check b/test/files/neg/sammy_overload.check
index 903d7c88f4..87b198f4f0 100644
--- a/test/files/neg/sammy_overload.check
+++ b/test/files/neg/sammy_overload.check
@@ -1,7 +1,7 @@
-sammy_overload.scala:11: error: missing parameter type for expanded function ((x$1: <error>) => x$1.toString)
-  O.m(_.toString) // error expected: eta-conversion breaks down due to overloading
-      ^
-sammy_overload.scala:12: error: missing parameter type
-  O.m(x => x) // error expected: needs param type
-      ^
-two errors found
+sammy_overload.scala:14: error: overloaded method value m with alternatives:
+  (x: ToString)Int <and>
+  (x: Int => String)Int
+ cannot be applied to (Int => Int)
+  O.m(x => x) // error expected: m cannot be applied to Int => Int
+    ^
+one error found