ADDITIONAL SYSTEM INFORMATION :
Recent macOS, JDK 12.

A DESCRIPTION OF THE PROBLEM :
I use generics to compute the upper bound type of generic varags parameters. Prior to JDK 12 the attached example compiled fine. Starting with JDK 12, javac shows errors.

REGRESSION : Last worked in version 11.0.2

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Copy the attached source code into a file BoundTest.java
2. javac BoundTest.java

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The example does compile without errors.
ACTUAL -
The compiler outputs the following error message:

BoundTest.java:4: error: incompatible types: cannot infer type-variable(s) T,R
        Number zero = a(
                       ^
    (argument mismatch; bad return type in lambda expression
      BigDecimal cannot be converted to INT#1)
  where T,R are type-variables:
    T extends Object declared in method <T,R>c(Function<T,R>)
    R extends Object declared in method <T,R>c(Function<T,R>)
  where INT#1,INT#2 are intersection types:
    INT#1 extends Number,Comparable<? extends INT#2>,Constable,ConstantDesc
    INT#2 extends Number,Comparable<?>,Constable,ConstantDesc
1 error


---------- BEGIN SOURCE ----------
interface BoundTest {

    default void shouldCompile() {
        Number zero = a(
                b(0),
                b(0.0),
                c(ignored -> java.math.BigDecimal.ZERO)
        );
    }

    <R> X<R> b(R r);

    <T, R> X<R> c(java.util.function.Function<T, R> f);

    @SafeVarargs
    static <R> R a(X<R>... xs) { return null; }

    interface X<R> {}

}

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

FREQUENCY : always