[Issue 21872] New: Warning: statement is not reachable should be error instead of warning!

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21872

          Issue ID: 21872
           Summary: Warning: statement is not reachable should be error
                    instead of warning!
           Product: D
           Version: D2
          Hardware: x86
                OS: Mac OS X
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: mingwu at gmail.com

Currently the compiler will *stop* after issue this warning:

```
Warning: statement is not reachable
```


This should be made as error, instead of warning.

warning: compiler continue to work.
error:   compiler stop.

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