[Issue 21317] New: Copy constructor defined but blitting still occurs

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21317

          Issue ID: 21317
           Summary: Copy constructor defined but blitting still occurs
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: eyal at weka.io

When defining a copy constructor, lack of NRVO (#21316) can still cause
implicit blitting, despite the DIP[1] claiming: "When a copy constructor is
defined for a struct, all implicit blitting is disabled for that struct".

For example, this program (same as #21316 but with a copy constructor):

  import std.stdio;

  struct S {
      this(int x) { writefln("%s", &this); }
      ~this() { writefln("%s", &this); }

      this(scope ref return S) {}
  }

  unittest {
      auto f() { return S(1); }
      auto x = f();
  }

Still does blit/copy, silently.

[1] https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1018.md

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