Passing aggregate types into templates by alias drops qualifier

[dlangBugzillaToGithub]

Andrei Alexandrescu (@andralex) reported this on 2020-05-25T18:30:05Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20863

CC List

• Boris Carvajal
• Adam D. Ruppe
• John Colvin
• Max Samukha
• Nick Treleaven (@ntrel)
• Steven Schveighoffer (@schveiguy)

Description

Took me a while to make this 100% self-contained. Both asserts should pass. The second fails.

alias AliasSeq(Seq...) = Seq;

template staticIndexOf(alias T, TList...)
{
    static if (TList.length == 0)
    {
        enum int staticIndexOf = -1;
    }
    else static if (is(T == TList[0]))
    {
        enum int staticIndexOf = 0;
    }
    else
    {
        private enum int t = staticIndexOf!(T, TList[1 .. $]);
        enum int staticIndexOf = t < 0 ? t : t + 1;
    }
}

struct Foo {}

static assert(staticIndexOf!(Foo, AliasSeq!(Foo)) == 0); // pass
static assert(staticIndexOf!(immutable(Foo), AliasSeq!(immutable(Foo))) == 0); // fail

[dlangBugzillaToGithub]

moonlightsentinel commented on 2020-05-25T20:06:33Z

Even smaller:

template staticFind(alias T, TList)
{
    enum bool staticFind = is(T == TList); // true;
}

static assert(staticFind!(Object, Object)); // pass
static assert(staticFind!(immutable Object, immutable Object)); // fail

[dlangBugzillaToGithub]

destructionator commented on 2020-05-25T23:07:21Z

I'm actually not sure if this is a bug or not.

But I didn't even know literal values were allowed to be passed to aliases, so I'm apparently behind the times!

But the spec isn't clear on if it takes built-in types at all, and is also underspecified on if it actually accepts a type or a symbol.

My thought is if it takes a symbol, the immutable qualifier might get legitimately dropped because that's part of the local type instead of the global symbol.

Of course it is more useful to just define it to take the whole type and be the same thing! But we should clarify the spec as well as the implementation to make it clear this is what is supposed to happen.

[dlangBugzillaToGithub]

maxsamukha commented on 2020-05-26T06:35:51Z

This is the same as or related to bug 2704.

[dlangBugzillaToGithub]

maxsamukha commented on 2020-05-26T07:05:06Z

(In reply to Max Samukha from comment #3)
> This is the same as or related to bug 2704.

Link: https://issues.dlang.org/show_bug.cgi?id=2704

[dlangBugzillaToGithub]

uplink.coder commented on 2020-05-26T07:45:33Z

The issue is not quite correct

struct Foo {}

template P(alias X)
{
    pragma(msg, X);
    enum P = 1;
}

int y = P!(const int);
int x = P!(immutable(double));
int z = P!(immutable(Foo));
int z2 = P!(immutable Foo);

The output of this is
const(int)
immutable(double)
Foo

What happens is that the types which are not symbols are merely "losslessly cast" to a symbol.
Therefore the STC is not dropped

[dlangBugzillaToGithub]

uplink.coder commented on 2020-05-26T07:51:18Z

struct Foo {}

template P(alias X)
{
    pragma(msg, X);
    enum P = 1;
}

int y = P!(const int);
int x = P!(immutable(double));
int z = P!(immutable(Foo));
int z2 = P!(immutable Foo);

The output of this is
const(int)
immutable(double)
Foo

What happens is that the types which are not symbols are merely "losslessly cast" to a symbol.
Therefore the STC is not dropped.

That means the new addition of types matching to alias behaves inconsistently.
Not the other way around.

[dlangBugzillaToGithub]

boris2.9 commented on 2020-05-30T13:33:30Z

The issue 13343 is also related.

[dlangBugzillaToGithub]

nick (@ntrel) commented on 2020-06-04T16:03:49Z

This bug is blocking the removal of template overloads in std.meta:
https://github.com/dlang/phobos/pull/7513

[dlangBugzillaToGithub]

boris2.9 commented on 2020-06-06T17:50:14Z

*** Issue 20899 has been marked as a duplicate of this issue. ***

[dlangBugzillaToGithub]

dlang-bot commented on 2020-06-24T11:53:08Z

@ntrel created dlang/dmd pull request #11320 "Fix Issue 20863 - Passing aggregate types by alias parameter drops qualifier" fixing this issue:

- Fix Issue 20863 - Passing aggregate types by alias param drops qualifier

https://github.com/dlang/dmd/pull/11320

[dlangBugzillaToGithub]

dlang-bot commented on 2020-07-08T09:03:18Z

dlang/phobos pull request #7542 "std.meta: Fix wrong assert for Issue #20863" was merged into master:

- af629b153b1eb93f201c6ef48dc5006b0e32882e by Nick Treleaven:
  std.meta: Comment out (fix) wrong assert for Issue #20863
  
  Necessary for https://github.com/dlang/dmd/pull/11320

https://github.com/dlang/phobos/pull/7542