Qualifier dropped from return type when AliasSeq is indexed with template argument

[dlangBugzillaToGithub]

Paul Backus (@pbackus) reported this on 2024-11-20T16:34:07Z

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

CC List

• Nick Treleaven (@ntrel)

Description

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.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2024-11-21T18:07:12Z

Reduced:

struct S
{
    int n;
    ref int get() const => n;
}

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

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2024-11-21T18:12:36Z

>     ref int get() const => n;

Sorry, you're right, ignore comment 1. That should be `ref const(int)`.

It's weird as the function prototype type is OK:

    ref const(AliasSeq!(int)[0]) g() const;
    pragma(msg, typeof(g)); // const ref const(int)()

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2024-11-21T18:16:09Z

OK, making the prototype a template again shows the wrong return type:

    ref const(AliasSeq!(int)[i]) get(size_t i)() const;
    pragma(msg, typeof(get!0)); // const ref int()