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

[d-bugmail at puremagic.com]

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

--- Comment #5 from johanengelen at weka.io ---
(In reply to RazvanN from comment #1)
> Well, the struct defines the member, it's just that it is disabled. One
> might argue that the output is correct and the programmer needs to check if
> the member is disabled or not

I strongly disagree.

The `@disabled` function line does not _define_ the function (i.e. there is no
implementation for it), at most it could be a _declaration_. But in fact, in my
opinion, the `@disable` line explicitly says that it's not even declared,
similar to C++'s "= delete".

Currently, adding `@disable this(this)` to a struct that normally wouldn't have
a postblit, actually makes the compiler emit a postblit function:
https://d.godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(j:1,lang:___d,source:'struct+S+%7B%0A++++int+i%3B%0A%0A++++ at disable+this(this)%3B%0A%7D'),l:'5',n:'0',o:'D+source+%231',t:'0')),k:34.254797922596914,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:ldc180,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:'0',trim:'0'),lang:___d,libs:!(),options:'-O3',source:1,wantOptInfo:'1'),l:'5',n:'0',o:'ldc+1.8.0+(Editor+%231,+Compiler+%231)+D',t:'0')),k:32.41186874406977,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:dmd20790,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:'0',trim:'0'),lang:___d,libs:!(),options:'-O',source:1),l:'5',n:'0',o:'dmd+2.079.0+(Editor+%231,+Compiler+%232)+D',t:'0')),k:33.33333333333333,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4

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