[Issue 18628] New: @disable this(this) erroneously adds `__postblit` member

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18628

          Issue ID: 18628
           Summary: @disable this(this) erroneously adds `__postblit`
                    member
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: johanengelen at weka.io

Testcase:
```
import std.traits;
import std.stdio;

void main() {
    static struct S { @disable this(this); }

    writeln(hasMember!(S, "__postblit")); // expected: false
    writeln(hasElaborateCopyConstructor!S); // expected: false
}
```

The program prints twice "true", instead of "false".
Note that `hasElaborateCopyConstructor` does a `hasMember!(S, "__postblit")`
check, so that's the root cause of the problem.

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