ADDITIONAL SYSTEM INFORMATION :
Linux 4.15.0-180-generic #189-Ubuntu
javac 17.0.3

A DESCRIPTION OF THE PROBLEM :
The following snippet does not compile with javac:

class Instanceof {
    static void doesNotWork(Object o) {
        if (o == null) {
            throw new Error();
        } else if (!(o instanceof String s)) {
            throw new Error();
        }
        System.out.println(s); // error here
    }
}

cannot find symbol
symbol: variable s
location: class Instanceof

According to the specification it seems that s should be visible after the if block.
I believe this is a bug in javac.
FYI, the ecliple compiler does accept this code.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a file named Instanceof.java with contents:
---
class Instanceof {
    static void doesNotWork(Object o) {
        if (o == null) {
            throw new Error();
        } else if (!(o instanceof String s)) {
            throw new Error();
        }
        System.out.println(s); // error here
    }
}
---
Attempt to compile this file using javac.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The file compiles.
ACTUAL -
The file does not compile.
The error is "cannot find symbol".

---------- BEGIN SOURCE ----------
class Instanceof {
    static void doesNotWork(Object o) {
        if (o == null) {
            throw new Error();
        } else if (!(o instanceof String s)) {
            throw new Error();
        }
        System.out.println(s); // error here
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Using an explicit else block solves the problem
   if (o == null) {
        throw new Error();
    } else if (!(o instanceof String s)) {
        throw new Error();
    } else {
        System.out.println(s);
    }
as does not using else if at all.
    if (o == null) {
        throw new Error();
    }
    if (!(o instanceof String s)) {
        throw new Error();
    }
    System.out.println(s);
It also works with exactly one if.
    if (o == null || !(o instanceof String s)) {
        throw new Error();
    }
    System.out.println(s);

FREQUENCY : always