• Andrei Alexandrescu (25/25) Apr 27 2021 Here's an example reduced from a more complex use case:
• Mathias LANG (17/19) Apr 28 2021 Not a bug, since `static foreach` does not introduce a scope,
• Mitacha (12/32) Apr 28 2021 Looks like alias reassignment is supported since this

Andrei Alexandrescu <SeeWebsiteForEmail erdani.org> writes:

Here's an example reduced from a more complex use case:

import std.meta : AliasSeq, staticMap;
template AliasThisType(T) {
     alias A = AliasSeq!();
     static foreach (U; AliasSeq!(__traits(getMember, T, "xyz"))) {
         alias A = AliasSeq!(A, typeof(U));
     }
     alias AliasThisType = A;
}

struct HasNoAliasThis {
}

struct HasAliasThis {
     HasNoAliasThis xyz;
     alias xyz this;
}

alias X = AliasThisType!HasAliasThis;

This should work, shouldn't it? When trying to compile I get:

test.d(7): Error: alias 
`test.AliasThisType!(HasAliasThis).__anonymous.A` conflicts with alias 
`test.AliasThisType!(HasAliasThis).A` at test.d(4)
HasNoAliasThis
test.d(20): Error: template instance `test.AliasThisType!(HasAliasThis)` 
error instantiating

Is this a bug? If not, where is the mistake in the code? If so, what 
would be a workaround? Thanks!

Mathias LANG <geod24 gmail.com> writes:

On Wednesday, 28 April 2021 at 00:29:46 UTC, Andrei Alexandrescu 
wrote:
 Is this a bug? If not, where is the mistake in the code?

Not a bug, since `static foreach` does not introduce a scope, 
you're trying to declare twice the same alias at the same scope, 
the compiler rightfully flags it.

 If so, what would be a workaround? Thanks!

Depends on what you want to do. Either use a different name, but 
it will fail as soon as you have more than one instantiation, or 
re-assign the alias:

```D
template AliasThisType(T) {
     alias A = AliasSeq!();
     static foreach (U; AliasSeq!(__traits(getMember, T, "xyz"))) {
         A = AliasSeq!(A, typeof(U)); // No more `alias` here
     }
     alias AliasThisType = A;
}
```

Mitacha <mateusz.mitaszka gmail.com> writes:

On Wednesday, 28 April 2021 at 09:07:15 UTC, Mathias LANG wrote:
 On Wednesday, 28 April 2021 at 00:29:46 UTC, Andrei 
 Alexandrescu wrote:
 Is this a bug? If not, where is the mistake in the code?

 Not a bug, since `static foreach` does not introduce a scope, 
 you're trying to declare twice the same alias at the same 
 scope, the compiler rightfully flags it.

 If so, what would be a workaround? Thanks!

 Depends on what you want to do. Either use a different name, 
 but it will fail as soon as you have more than one 
 instantiation, or re-assign the alias:

 ```D
 template AliasThisType(T) {
     alias A = AliasSeq!();
     static foreach (U; AliasSeq!(__traits(getMember, T, 
 "xyz"))) {
         A = AliasSeq!(A, typeof(U)); // No more `alias` here
     }
     alias AliasThisType = A;
 }
 ```

Looks like alias reassignment is supported since this 
[PR](https://github.com/dlang/dmd/pull/11846)
Why use `static foreach` to get alias this memeber? You could use 
something like this:
```D
template AliasThisType(T) {
     alias AliasThisMember = __traits(getAliasThis, T);
     alias AliasThisType = typeof(__traits(getMember, T, 
AliasThisMember));
}
```