Why can't diamond infer types on anonymous inner classes?

Question

In Java 7 and later, diamond can be used to infer types on normally like so without an issue:

List<String> list = new ArrayList<>();

However, it can't for anonymous inner classes like this:

List<String> st = new List<>() { //Doesn't compile

    //Implementation here

}

Why is this? Logically in this scenario, I can definitely infer the type as String. Is there a logical reason for this decision where the type cannot actually be inferred on anonymous inner classes, or was it omitted for other reasons?

Answer 1

In the JSR-334:

Using diamond with anonymous inner classes is not supported since doing so in general would require extensions to the class file signature attribute to represent non-denotable types, a de facto JVM change.

What I guess is that as everybody knows, anonymous class leads to a generation of its own class file.

I imagine that generic type doesn't exist within these files and rather replaced by the effective (static) type (thus declared by the explicit type like <String> at declaration object time).

Indeed, file corresponding to an inner class is never shared across multiple different instantiations of it, so why bother with generics into it?! :).

It would be more hardly achievable (and surely useless) for compiler to force an extension (by adding a special attribute for generics) to theses kind of class files.