Bug in compiler?

[ketmar via Digitalmars-d]

On Mon, 15 Dec 2014 16:01:35 +0200
Shachar Shemesh via Digitalmars-d <digitalmars-d at puremagic.com> wrote:

> Please consider the following program:
> import std.exception;
> 
> void main()
> {
>      struct A {
>          int a;
> 
>          @disable this(this);
>          @disable ref A opAssign(const ref A);
> 
>          ref A opOpAssign(string op: "~")(int data) {
>              a += data;
> 
>              return this;
>          }
>      }
> 
>      auto a = A(2);
> 
>      a ~= 3;
> 
>      assertThrown!Exception(a ~= 3);
> }
> 
> Compilation (dmd 2.066.1) fails on the assertThrown line:
> Error: struct test.main.A is not copyable because it is annotated with 
> @disable
> 
> What I do not understand is why A should need to be copyable. Where is 
> the copy made? I'm guessing this is because of the lazy definition of 
> the expression, but still I don't see any reason to create a copy.

yes, this is due to `lazy` in assrtThrown. what `lazy` does is actually
creating a lambda. then compiler tries to deduce the type of the
expression and it got `A`. so it generates the code like
`A expression`. it can't see `ref` there, as `a` type is `A`, not
`ref A`.

you can workaround that with creating the necessary lambda manually:

  assertThrown!Exception((ref A aa) { aa ~= 3; }(a));


tl;dr: no, it's not a bug in the compiler. but the error message is
misleading and you are required to understang some internals to make
sense out of it.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 181 bytes
Desc: not available
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20141215/e9a3d029/attachment.sig>