Imported From: https://issues.scala-lang.org/browse/SI-10072?orig=1
Reporter: Kamil Kloch (kamilk)
Affected Versions: 2.12.0

@lrytz said:
For me, the snippet doesn't compile in 2.11.8. I actually get two errors in 2.11.8:

Test.scala:3: error: type mismatch;
 found   : (Double, <empty>.T) => Double
 required: (Double, T(in trait Foo)) => Double
    foldLeft(num.zero)(num.plus)
                           ^
Test.scala:3: error: type mismatch;
 found   : B(in method sum$mcD$sp)
 required: <empty>.B
    foldLeft(num.zero)(num.plus)
                      ^

Kamil Kloch (kamilk) said:
@lukasz Rytz: it does compile on 2.11.8
https://github.com/kamilkloch/SI-10072

@lrytz said:
now it works for me too - i have no idea what i did wrong before..

@lrytz said (edited on Dec 6, 2016 8:06:51 PM UTC):
Intermediate result: this is related to #5284, which was fixed in scala/scala#755, i still have to spend some time to understand that change.

The example in this ticket compiles using -Ydelambdafy:inline, in which case the castType method introduced in the above PR casts the function instantiation (new anonfun) to the necessary type (Int, A) => Int.

This cast is not performed under -Ydelambdafy:method, where the Function survives until specialization and is not replaced by new anonfun. So "duplicators" attempts to re-typecheck (x: B, y: B) => $anonfun$sum(x, y) with expected type (Int, A) => Int, which then yields the type mismatch.

@lrytz said:
scala/scala#5587