Summary
-------

According to the `JLS 24`, lambda expressions are compatible in an assignment context, invocation context, or casting context with a target type `T` if `T` is a functional interface type. Javac is allowing the type of lambda expressions to be classes in some cases. We are proposing fixing this issue by synchronizing the javac compiler with the `JLS 24`.

Problem
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Javac is not in sync with the `JLS 24`, in particular the Javac compiler is allowing the type of lambda expressions to be classes in some cases.

For example this code is accepted by javac:

    class Test {
        void m() {
            Test r = (Test & Runnable) () -> System.out.println("Hello, World!");
        }
    }

here `Test&Runnable` is an intersection type but as we will see below in the referred specification sections a notional functional interface can't be induced from it as one of its components is a class different from `java.lang.Object`

Solution
--------

Synchronize the Javac compiler with the `JLS 24`. The compiler should not allow the type of lambda and method reference expressions to be classes.

Specification
-------------

This CSR is not proposing any specification change. But there is some normative specification text related to it. We are referring to section `§15.27.3 Type of a Lambda Expression` of the `JLS 24` in particular where it reads:

    A lambda expression is compatible in an assignment context, invocation context,
    or casting context with a target type T if T is a functional interface
    type (§9.8) and the expression is congruent with the function type of the
    ground target type derived from T .

Then from `§9.8 Functional Interfaces` we have that:

    The declaration of a functional interface allows a functional interface type to be
    used in a program. There are four kinds of functional interface type:
    • An intersection type (§4.9) that induces a notional functional interface

On the other hand section `§4.9 Intersection Types` says:

    Every intersection type T 1 & ... & T n induces a notional class or interface for the
    purpose of identifying the members of the intersection type, as follows:
    (...)
    • If C k is Object , a notional interface is induced; otherwise, a notional class
    is induced with direct superclass type C k . This class or interface has direct
    superinterface types T 1 ', ..., T n ' and is declared in the package in which the
    intersection type appears.