Cannot partially destructure type qualifiers with == form of IsExpression

[dlangBugzillaToGithub]

Meta reported this on 2014-11-22T22:55:26Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=13767

CC List

• Vladimir Panteleev (@CyberShadow)

Description

The following code fails to print anything, although I believe it should be valid and should print `const(int)`.

shared const int i;

static if (is(typeof(i) == shared U, U))
{
    pragma(msg, U);
}

void main()
{
}

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2017-07-02T01:52:30Z

No, because you use ==, which does exact matching. If you want to partially match the type (i.e. check if it's implicitly convertible to shared), use `:`, not `==`:

shared const int i;

static if (is(typeof(i) == shared U, U))
{
    pragma(msg, U); // now prints const(int)
}

void main()
{
}

[dlangBugzillaToGithub]

monkeyworks12 commented on 2017-07-02T04:37:47Z

I'm assuming in your example you meant to use : instead of ==.

That aside, why shouldn't this work? I'm trying to destructure some type which matches the pattern `shared U`. `shared const(int)` *should* match that pattern. What I'm trying to express is `∃ U: shared U == shared const int`; it seems incorrect to me that that I should have to use the subtyping form of is.

[dlangBugzillaToGithub]

dlang-bugzilla (@CyberShadow) commented on 2017-07-02T10:14:49Z

(In reply to monkeyworks12 from comment #2)
> I'm assuming in your example you meant to use : instead of ==.

Oops, yes.

> That aside, why shouldn't this work? I'm trying to destructure some type
> which matches the pattern `shared U`. `shared const(int)` *should* match
> that pattern. What I'm trying to express is `∃ U: shared U == shared const
> int`; it seems incorrect to me that that I should have to use the subtyping
> form of is.

OK, I see what you mean now. It looks like partial destructuring of type qualifiers was never implemented for == variants of IsExpression. It is the same for inout:

static assert(!is(shared const int == shared U, U)); // should work
static assert( is(shared const int :  shared U, U));
static assert(!is(inout  const int == const  U, U)); // should work
static assert( is(inout  const int :  const  U, U));