[Issue 24869] New: Qualifier dropped from return type when AliasSeq is indexed with template argument

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24869

          Issue ID: 24869
           Summary: Qualifier dropped from return type when AliasSeq is
                    indexed with template argument
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

As of DMD 2.109.1, the following program fails to compile:

---
alias AliasSeq(Args...) = Args;

struct S
{
    int n;
    ref const(AliasSeq!(int)[i]) get(size_t i)() const => n;
}

void main()
{
    const S s;
    s.get!0;
}
---

The error message is:

---
bug.d(6): Error: expression `this.n` of type `const(int)` is not implicitly
convertible to return type `ref int`
bug.d(12): Error: template instance `bug.S.get!0LU` error instantiating
---

As show in the error message, the compiler has erroneously determined the
return type of `get!0` to be `int`, when it should instead be `const(int)`.

The same error also occurs with the immutable, inout, and shared qualifiers.

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