FULL PRODUCT VERSION :
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux 4.4.0-66-generic #87-Ubuntu SMP Fri Mar 3 15:29:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
The following code does not compile:

import java.util.Optional;

class B<T> {}

public class Test {
static B<Integer> fn(B<Object> a) { return null; }

public static void main(String[] args) {

                Optional<B> opt = Optional.empty(); // note raw type parameter B

                B<Integer> res = opt.map(Test::fn).get(); // COMPILE ERROR: incompatible types: Object cannot be converted to B<Integer>

}
}

It looks like javac infers the result of opt.map(Test::fn) to be Optional<Object> instead of Optional<B<Integer>>. I expected it to be inferred as Optional<B<Integer>> because fn returns B<Integer>.

Also, it's curious that the program does compile if we explicitly cast the result of opt.map as follows:

B<Integer> res = ((Optional<B<Integer>>) opt.map(Test::fn)).get();

It also compiles successfully if we make fn take a raw B, instead of B<Object>.

If the signature of fn is erased because of unchecked conversion, shouldn't the erasure return a raw B? Does the compiler erase the return to Object, or why does the compilation fail?


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the program shown below

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Expected program to compile successfully
ACTUAL -
Compilation error

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Compilation error: incompatible types: Object cannot be converted to B<Integer>

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.Optional;

class B<T> {}

public class Test {
static B<Integer> fn(B<Object> a) { return null; }

public static void main(String[] args) {

                Optional<B> opt = Optional.empty(); // note raw type parameter B

                B<Integer> res = opt.map(Test::fn).get(); // COMPILE ERROR: incompatible types: Object cannot be converted to B<Integer>

}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Multiple workarounds:

0) Don't use raw types in the first place

1) declare fn() as: static B<Integer> fn(B a) { return null; } // NOTE: parameter is raw B instead of B<Object>

2) explicit cast: B<Integer> res = ((Optional<B<Integer>>) opt.map(Test::fn)).get();