[Issue 19794] const on AliasSeq ignored

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19794

Basile-z <b2.temp at gmx.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |accepts-invalid
           Priority|P4                          |P3
                 CC|                            |b2.temp at gmx.com
           Hardware|x86_64                      |All
                 OS|Linux                       |All
           Severity|enhancement                 |normal

--- Comment #3 from Basile-z <b2.temp at gmx.com> ---
Not and ER, that is well a bug, according to
https://dlang.org/spec/const3.html#const_type

> [...]. Each applies transitively to all subtypes.

The problem is that the DMD class `TypeTuple` (in mtype.d) does not override
`.constOf`, `immutableOf()`, etc.

Tagging as accept-invalid given that

```d
alias AliasSeq(T...) = T;

const(AliasSeq!(int, int)) a;

void main()
{
    a[0] = 1;
}    
```

should be reject just like when `const` is variable storage class.

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