There is a weird rule on how compiler treats alias this for the N and S types bellow.

As you can see, somehow S losses it's type and ends up a plain string withing the Algebraic type. I would expect that all types should be the treated the same, why a string alias will be different then a bool or a double?

int main(string[] argv)
{
	import std.variant	: Algebraic;

	struct N { double val; alias val this; }
	struct S { string val; alias val this; }
	alias T = Algebraic!(N, S);
	pragma(msg, T.AllowedTypes); // prints (N, string)??

	T t = N(1.0);
	assert(t.get!N == 1.0); // works
	// t = 0.4; // fails, OK, variant.d(586): Error: static assert  "Cannot store a double in a VariantN!(8u, N, S). Valid types are (N, string)"
	// t = S("foo"); // this fails, why? Error: static assert  "Cannot store a S in a VariantN!(8u, N, S). Valid types are (N, string)"
	t = "bar"; // this works... why?
	// assert(t.get!S == "bar"); //this fails Variant: attempting to use incompatible types immutable(char)[] and main.main.S at std\variant.d(1475)
	assert(t.get!string == "bar"); // works, why?
	
	return 0;
}

On Monday, 5 October 2015 at 11:31:32 UTC, Radu wrote:
> There is a weird rule on how compiler treats alias this for the N and S types bellow.
>
> [...]

Please file a bug report.

Also do the errors change if you reverse the order in T i.e. alias T = Algebraic!(S,N); ?

On Tuesday, 6 October 2015 at 06:37:16 UTC, Nicholas Wilson wrote:
> On Monday, 5 October 2015 at 11:31:32 UTC, Radu wrote:
>> There is a weird rule on how compiler treats alias this for the N and S types bellow.
>>
>> [...]
>
> Please file a bug report.
>
> Also do the errors change if you reverse the order in T i.e. alias T = Algebraic!(S,N); ?

OK, will do.

Nope alias T = Algebraic!(S,N) gives the same error. AllowedTypes are (string, N)

This happens in 2.068.2 btw

On Tuesday, 6 October 2015 at 06:37:16 UTC, Nicholas Wilson wrote:
> On Monday, 5 October 2015 at 11:31:32 UTC, Radu wrote:
>> There is a weird rule on how compiler treats alias this for the N and S types bellow.
>>
>> [...]
>
> Please file a bug report.
>
> Also do the errors change if you reverse the order in T i.e. alias T = Algebraic!(S,N); ?

bug report

https://issues.dlang.org/show_bug.cgi?id=15168