Lambda expression and method overloading doubts

Solution 1:

I think you found this bug in the compiler: JDK-8029718 (or this similar one in Eclipse: 434642).

Compare to JLS §15.12.2.1. Identify Potentially Applicable Methods:

…

A lambda expression (§15.27) is potentially compatible with a functional interface type (§9.8) if all of the following are true:

The arity of the target type's function type is the same as the arity of the lambda expression.

If the target type's function type has a void return, then the lambda body is either a statement expression (§14.8) or a void-compatible block (§15.27.2).

If the target type's function type has a (non-void) return type, then the lambda body is either an expression or a value-compatible block (§15.27.2).

Note the clear distinction between “void compatible blocks” and “value-compatible blocks”. While a block might be both in certain cases, the section §15.27.2. Lambda Body clearly states that an expression like () -> {} is a “void compatible block”, as it completes normally without returning a value. And it should be obvious that i -> {} is a “void compatible block” too.

And according to the section cited above, the combination of a lambda with a block that is not value-compatible and target type with a (non-void) return type is not a potential candidate for the method overload resolution. So your intuition is right, there should be no ambiguity here.

Examples for ambiguous blocks are

() -> { throw new RuntimeException(); }
() -> { while (true); }

as they don’t complete normally, but this is not the case in your question.

Solution 2:

This bug has already been reported in the JDK Bug System: https://bugs.openjdk.java.net/browse/JDK-8029718. As you can check the bug has been fixed. This fix syncs javac with the spec in this aspect. Right now javac is correctly accepting the version with implicit lambdas. To get this update, you need to clone javac 8 repo.

What the fix does is to analyze the lambda body and determine if it's void or value compatible. To determine this you need to analyze all return statements. Let's remember that from the spec (15.27.2), already referenced above:

A block lambda body is void-compatible if every return statement in the block has the form return.
A block lambda body is value-compatible if it cannot complete normally (14.21) and every return statement in the block has the form return Expression.

This means that by analyzing the returns in the lambda body you can know if the lambda body is void compatible but to determine if it's value compatible you also need to do a flow analysis on it to determine that it can complete normally (14.21).

This fix also introduces a new compiler error for cases when the body is neither void nor value compatible, for example if we compile this code:

class Test {
    interface I {
        String f(String x);
    }

    static void foo(I i) {}

    void m() {
        foo((x) -> {
            if (x == null) {
                return;
            } else {
                return x;
            }
        });
    }
}

the compiler will give this output:

Test.java:9: error: lambda body is neither value nor void compatible
    foo((x) -> {
        ^
Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output
1 error

I hope this helps.

Lambda expression and method overloading doubts

Solution 1:

Solution 2:

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