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First, I wasn't speaking about a general case, I was speaking about the case presented.

Second, as of Java 8, the compiler knows whether a local variable is "effectively final". In the example given, f qualifies as effectively final. In earlier versions, my point stands that it's simply because the compiler isn't sophisticated enough.

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The point I don't agree with is that it's really about the compiler. Had it been an optimization that doesn't affect which warnings or errors you get then yes, that's all the compiler. But what you need here is for the types of variables to incorporate flow analysis such that f gets compatible types on both reads if it doesn't change, and incompatible if it does. That would be an incredibly complex type system and people already struggle with how generics in java work.

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