Unknown error while instantiating: __traits(compiles) hides the real issue

[dlangBugzillaToGithub]

ilya.yanok reported this on 2024-11-13T12:03:09Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24857

CC List

• Luís Ferreira
• Nick Treleaven

Description

dustmite minimized example (probably could be minimized a bit further):
```
template AllMembersInModule(alias Module) {
    import std: AliasSeq, Filter;
    alias all_members = AliasSeq!(__traits(allMembers, Module));

    template CanAccess(string member) {
        enum CanAccess = __traits(compiles, __traits(getMember, Module, member));
    }
    alias x = Filter!(CanAccess, all_members);

    alias AllMembersInModule = y;
}

template t() {
    alias MODULE = __traits(parent, {});
    static foreach (M; AllMembersInModule!MODULE) {
        }
}

alias AllMembers = AllMembersInModule!(__traits(parent, {}));
mixin t;
```

The issue here is a typo: `y` should be `x`, but we get an unknown error instead.

If I remove `alias AllMembers` it works as expected.

BTW, if I remove `mixin t` line, it compiles without error, is it expected, are aliases lazy?

[dlangBugzillaToGithub]

nick commented on 2024-11-13T16:34:33Z

I couldn't reduce it any more than this:

```
template AllMembersInModule(alias Module) {
    import std.meta : Filter;
    alias all_members = __traits(allMembers, Module);

    template CanAccess(string member) {
        enum CanAccess = __traits(compiles, __traits(getMember, Module, member));
    }
    alias x = Filter!(CanAccess, all_members);

    alias AllMembersInModule = y;
}

alias AllMembers = AllMembersInModule!(__traits(parent, {}));
pragma(msg, AllMembers); // added
```

Error: unknown, please file report on issues.dlang.org
aliastypo.d(15):        while evaluating `pragma(msg, AllMembers)`