Array dup ignores copy constructor with qualified argument

[dlangBugzillaToGithub]

Paul Backus (@pbackus) reported this on 2024-03-08T15:15:00Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24432

Description

As of DMD 2.107.1, the following program fails to compile:

---
struct S
{
    int* dummy;
    this(ref immutable S) {}
}

void main()
{
    immutable(S) s1;
    S s2 = s1; // ok

    auto arr1 = [immutable(S)()];
    auto arr2 = arr1.dup; // error
}
---

The error message is:

---
bug.d(13): Error: none of the overloads of template `object.dup` are callable using argument types `!()(immutable(S)[])`
/usr/include/dmd/druntime/import/object.d(3007):        Candidates are: `dup(T : V[K], K, V)(T aa)`
/usr/include/dmd/druntime/import/object.d(3045):                        `dup(T : V[K], K, V)(T* aa)`
/usr/include/dmd/druntime/import/object.d(3835):                        `dup(T)(T[] a)`
  with `T = immutable(S)`
  must satisfy the following constraint:
`       !is(const(T) : T)`
/usr/include/dmd/druntime/import/object.d(3858):                        `dup(T)(const(T)[] a)`
  with `T = S`
  must satisfy the following constraint:
`       is(const(T) : T)`
---

dup should be able to create a mutable copy of an immutable S using the appropriately-qualified copy constructor. However, it fails to do so, because its template constraint is too strict.

This bug was originally reported in issue 20879.