[broken] Support `TypeQual(TypeSeq)`

[ntrel]

First commit for immutable(TypeSeq) seems to work. Second commit for const(TypeSeq) gets a bizarre result when indexed (see test). Any ideas why/alternative suggestions?

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22189	critical	type qualifier not applied to type tuple

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Testing this PR locally

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[atilaneves]

What is the point of an immutable AliasSeq?

[ntrel]

@atilaneves AFAIK a type sequence is a type. So without an implementation of this, do you think it's better to have a special case in the type system, that only type sequences can't have a type qualifier?

[atilaneves]

@atilaneves AFAIK a type sequence is a type. So without an implementation of this, do you think it's better to have a special case in the type system, that only type sequences can't have a type qualifier?

Compile-time programming is all immutable anyway. What am I missing?

[jmdavis]

@atilaneves AFAIK a type sequence is a type. So without an implementation of this, do you think it's better to have a special case in the type system, that only type sequences can't have a type qualifier?

Compile-time programming is all immutable anyway. What am I missing?

You can use the result of an AliasSeq to do stuff like declare variables, e.g. this fails right now, because the const is lost:

    import std.meta;
    alias Seq = AliasSeq!(int, float, string);
    alias CSeq = const Seq;

    CSeq[0] a;
    static assert(is(typeof(a) == const int));

    CSeq[1] b;
    static assert(is(typeof(b) == const float));

    CSeq[2] c;
    static assert(is(typeof(c) == const string));

So, if it doesn't work to use type qualifiers on an AliasSeq, that can cause problems, particularly with code generating types by doing type introspection and manipulating the results.

This may also relate (I'm not sure) to https://issues.dlang.org/show_bug.cgi?id=24516, which shows that type qualifiers get lost when aliasing the result of tupleof, which causes problems with stuff like templated traits, since they have to use alias for their result when operating on symbols or types.