Java 9+ invokedynamic string concatenation decompiled incorrectly: char printed as int, trailing operand dropped

[BoxiYu]

Procyon mistranslates StringConcatFactory (Java 9+) string concatenation, silently producing wrong output. The decompiled code compiles, so the defect is only visible at runtime.

Minimal reproduction (compiled with javac --release 11):

public class T {
    public static void main(String[] a) {
        char c = 'Z';
        System.out.println("c=" + c + "x");
    }
}

Procyon 0.6.0 decompiles the body to:

System.out.println("c=" + 90);

i.e. it constant-folds the char to its integer value 90 and drops the trailing + "x" operand.

• Expected output: c=Zx
• Actual output (after recompiling Procyon's source): c=90

CFR and Vineflower decompile the same class correctly. Found by round-trip testing (compile → decompile → recompile → compare output).