Result expressions in lambda bodies are required to be _assignable_ to the targeted return type. For a constant expression, this may involve a narrowing primitive conversion.

If the lambda expression is an invocation argument, and the target type mentions an inference variable, the current spec text requires _invocation compatibility_, which does not allow a narrowing primitive conversion.

Example:

interface ByteFunction<T> {
  byte apply(T arg);
}

<T> void m(ByteFunction<T> f) {}

ByteFunction<String> f = s -> 100; // no error
m(s -> 100); // expected: no error; specified: error

javac already seems to behave correctly here.