instanceof pattern matching should reject supertypes

[DasBrain]

The following snippet should be rejected by the compler:

String str = "foo";
if (str instanceof String s) {}

because String is a the same (or a supertype) of the type of the variable str, so this check would always succeed (if str != null).

This is mandated by JLS §15.20.2:

If the type of the RelationalExpression is a subtype of the type of the Pattern, then a compile-time error occurs.

[srikanth-sankaran]

Reproduced. I'll take a look. Thanks

[srikanth-sankaran]

JDK21 javac accepts the code above while upto JDK20, it was rejected.

[srikanth-sankaran]

JLS21 15.20.2 is missing the said clause - however there are phantom bullets: See https://bugs.openjdk.org/browse/JDK-8317695

[srikanth-sankaran]

Closing with no change for reasons cited - latest JLS and latest javac don't call for or enforce this. Should there be a change in future, we will revisit