[Issue 24842] New: No ability to overload unary logial not operator

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24842

          Issue ID: 24842
           Summary: No ability to overload unary logial not operator
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: info at lapyst.by

Dlang has wide range support for overloading nearly all operators the language
supports. However, I'm missing support for the unary logical not operator.

Usecases are implementation of (feature-)flags, or my current use-case,
notation of if an certain format is allowed or not for serialization /
processing. Essentially all datastuctures that are logically negateable would
benefit of the ability to overload the logical not operator.

Example:
```d
struct FormatId {
    string id;
    bool allowed = true;

    this(string id) {
        this.id = id;
    }

    private this(string id, bool allowed) {
        this.id = id;
        this.allowed = allowed;
    }

    auto opUnary(string op)() if (op == "!") {
        return FormatId(this.id, !this.allowed);
    }
}

unittest {
  auto f1 = FormatId("test");
  assert(f1.allowed == true);
  auto f2 = !f1;
  assert(f1.allowed == true);
  assert(f2.allowed == false);
}
```

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