Wrong choice of generic mutable/const/immutable methods

[dlangBugzillaToGithub]

Илья Ярошенко reported this on 2017-08-20T05:19:27Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=17766

CC List

• ag0aep6g
• ZombineDev

Description

static struct Foo()
    {
        char opIndex(Indexes...)(Indexes indexes) immutable
        {
            return 'i';
        }

        char opIndex()() const
        {
            return 'c';
        }

        char opIndex(Indexes...)(Indexes indexes)
        {
            return 'm';
        }
    }
    pragma(msg, Foo!()()[]);
    pragma(msg, (cast(const) Foo!()())[]);
    pragma(msg, (cast(immutable) Foo!()())[]);

Prints
'c'
'c'
'c'

But shuold be

'm'
'c'
'i'

Type qualifier should have priority.

[dlangBugzillaToGithub]

alexandru.ermicioi commented on 2018-10-22T19:11:27Z

This is not the only case, here is another case of wrong selection:

----------------------
class Mquartz(T, Z) {
    Z pass(T component) const {
        "Mutable pass".writeln;
        return component.z;
    }
    const(Z) pass(const T component) const {
        "Const pass".writeln;
        return component.z;
    }
    const(Z) pass(immutable T component) const {
        "Immutable pass".writeln;
        return component.z;
    }
    const(Z) pass(inout T component) const {
        "Inout pass".writeln;
        return component.z;
    }
}

struct V {
    int z;
}

void main() {

    auto m = new Mquartz!(V, typeof(V.z));

    // V v = V(21);
    // writeln(m.pass(v));
    writeln(m.pass(V(20)));
    writeln(m.pass(const(V)(20)));
    writeln(m.pass(immutable(V)(20)));
    writeln(m.pass(inout(V)(20)));
}
-------------------------
Current logic will select only const version with const argument instead of selecting right ones:
-------------------------
Const pass
20
Const pass
20
Const pass
20
Const pass
20
-------------------------
The overload selection logic works as expected only if m is const, then response will as expected:
-------------------------
Mutable pass
20
Const pass
20
Immutable pass
20
Inout pass
20
-------------------------