FULL PRODUCT VERSION :
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 10.0.16299.248]

A DESCRIPTION OF THE PROBLEM :
If a user mistakely invokes CompletionStage.thenApply() with a Supplier instead of with a Function then the compiler gives a misleading error message.

Given:

CompletionStage.thenApply(() -> methodThatReturnsCompletionStage())

I get:

method thenApply in interface java.util.concurrent.CompletionStage<T> cannot be applied to given types;
required: java.util.function.Function<? super java.util.Map<com.holdmyspot.pyramid.exchange.Currency,java.math.BigDecimal>,? extends U>
found: ()->string[...]coin)
reason: cannot infer type-variable(s) U
(argument mismatch; incompatible parameter types in lambda expression)

The problem has nothing to do with type-parameter U, but rather it is type-parameter T that has to be changed. Instead of:

  CompletionStage.thenApply(() -> methodThatReturnsCompletionStage())
  
the code should read:

  CompletionStage.thenApply(ignored -> methodThatReturnsCompletionStage())
  
Please improve this error message.


REPRODUCIBILITY :
This bug can be reproduced always.