[Issue 21256] New: The toString() in Appender will crash the app

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21256

          Issue ID: 21256
           Summary: The toString() in Appender will crash the app
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: critical
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: bitworld at qq.com

Here is the simple test code:

import std.array;
import std.stdio;

void main() {
    Appender!string sb;
    writeln(sb.toString()); // crash here
}

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