[Issue 20149] [DIP1000] Local data escapes `inout` method if not decorated with `return`

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20149

--- Comment #25 from Walter Bright <bugzilla at digitalmars.com> ---
A further simplified test case:

  @safe:

  struct S {
  //inout(char)* slice() inout return /* gives correct error */
    inout(char)* slice() inout          /* no error */
    {
        return &buf;
    }

    char buf;
  }

  char* fun() {
    S sb;
    return sb.slice();
  }

Note that if slice() were made as a static function, with sb passed by ref as
the first parameter, the compilation gives the correct error. Hence it's
related to the `this` being a special case.

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