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digitalmars.D.learn - IsTuple returns true for Nullable!SomeTuple

reply Ben Jones <fake fake.fake> writes:
This seems like very surprising behavior to me.  Is it a bug?

import std.typecons;
alias NT = Nullable!(Tuple!(int, double));
pragma(msg, isTuple!NT); //prints true!
Dec 01 2020
parent reply Paul Backus <snarwin gmail.com> writes:
On Wednesday, 2 December 2020 at 05:25:09 UTC, Ben Jones wrote:

 This seems like very surprising behavior to me.  Is it a bug?

 import std.typecons;
 alias NT = Nullable!(Tuple!(int, double));
 pragma(msg, isTuple!NT); //prints true!


No, this is not a bug, because Nullable!T currently has an 
implicit conversion to T via `alias this`. [1] However, this 
implicit conversion is deprecated, and will be removed in a 
future release. Once that happens, `isTuple!NT` will be `false`, 
as you'd expect.

[1] 
http://phobos.dpldocs.info/std.typecons.Nullable.1.html#alias-this
Dec 02 2020
parent Ben Jones <fake fake.fake> writes:
On Wednesday, 2 December 2020 at 12:59:52 UTC, Paul Backus wrote:

 No, this is not a bug, because Nullable!T currently has an 
 implicit conversion to T via `alias this`. [1] However, this 
 implicit conversion is deprecated, and will be removed in a 
 future release. Once that happens, `isTuple!NT` will be 
 `false`, as you'd expect.

 [1] 
 http://phobos.dpldocs.info/std.typecons.Nullable.1.html#alias-this


I guess it's a moot point because the implicit conversion is 
deprecated, but it seems strange to me that isTuple behaves like 
canBeUsedAsTuple.

Seems like the reasonable thing to do with a 
Nullable!(Tuple!Whatever) would be to check 
isTuple!(TemplateArgsOf!MyNullable)

Thanks for the response
Dec 02 2020