[Patterns] Inappropriate diagnostic with type test patterns

[srikanth-sankaran]

The following program generates a diagnostic: Type String cannot be safely cast to Example<String> that is misleading. This is not a situation that involves unsafe generic cast. This is a patent type incompatibility error. Javac reports: error: incompatible types: String cannot be converted to Example<String>.

class Example<T> {
	private void foo(String x) {
		if (x instanceof Example<String> es) {
			
		}
	}
}

[srikanth-sankaran]

If I add this statement to the snippet above:

Example<String> es = x;

I get: Type mismatch: cannot convert from String to Example<String> which is bang on.