FULL PRODUCT VERSION :
java version "10" 2018-03-20
Java(TM) SE Runtime Environment 18.3 (build 10+46)
Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.16299.334]

A DESCRIPTION OF THE PROBLEM :
1. Compile testcase
2. Exception thrown:

error: incompatible types: bad return type in lambda expression
        return () -> test().thenCompose(value -> {
    null

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The error message should indicate that the lambda was expected to return a CompletionStage<Integer> but instead it returned a Supplier<CompletionStage<Integer>>
ACTUAL -
Error message cut off with "null" apparently due to an internal error

REPRODUCIBILITY :
This bug can be reproduced always.