On 12.07.2018 15:29, Andrei Alexandrescu wrote:
On 07/11/2018 05:55 AM, Nick Treleaven wrote:
...

Removing `static` works. Otherwise I tried changing `ref` to `alias`:

Error: variable src cannot be read at compile time

But this shorter code seems to work fine:

this.tupleof = src.tupleof;

Odd. Timon, what would be the reason for that error? Razvan, can you please look into removing "static" for now. Thanks!

The reason for this specific error is that `src.i` is neither a symbol nor a constant value. tupleof is a case of built-in compiler magic, because it can produce an expression tuple that contains run-time values that are not symbols. The following (manually unrolled) code also does not work:

alias field0 = s.tupleof[0];
t.tupleof[0] = field0;
alias field1 = s.tupleof[1];
t.tupleof[1] = field1;
alias field2 = s.tupleof[2];
t.tupleof[2] = field2;

Error: alias `a` cannot alias an expression `tuple(s.a, s.b, s.c)[0]`
Error: alias `b` cannot alias an expression `tuple(s.a, s.b, s.c)[1]`
Error: alias `c` cannot alias an expression `tuple(s.a, s.b, s.c)[2]`

It could be argued that the `static foreach` implementation should produce the same error message. (The fact that the AliasDecl constructor takes a Dsymbol instead of an Expression has led to multiple reimplementations of parts of the aliasing logic in different parts of the DMD code, `static foreach` is using the same implementation also used by unrolled foreach, and it requires that all loop variables are either symbols or constant values, while unrolled foreach can fall back to introducing runtime variables for this special case.)

One way to fix is to lift all the unnecessary limitations that are introduced by the fact that the AliasDecl constructor takes a Dsymbol. I.e. that "you can only alias symbols".

Alternatively, it would be possible to redefine `static foreach` statements such that they work for any aggregate with statically known length and element types, and to allow run-time loop variables to be generated when iterating over run-time values.

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