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digitalmars.D.learn - Template Mixin issue

reply Mike L. <mike.linford gmail.com> writes:
Hello,

I was wondering why the following does not work:

interface A(Type)
	{ Type blah(); }

template ADefaults(Type, AType : A!(Type))
{
	Type blah()
		{ return Type.init; }
}

class B(Type) : A!(Type)
	{ mixin ADefaults!(Type, B!(Type)); }

void main()
	{ auto x = new B!(int)(); }

It gives the error: test.d(11): mixin ADefault!(int,B) does not match any
template declaration

However, it will compile if you change the ADefaults header to:

template ADefaults(Type, AType)

If the compiler can tell that B!(int) is a type, why can't it tell that it is a
child class of A!(int) ?
Jan 29 2009
parent reply Christopher Wright <dhasenan gmail.com> writes:
Mike L. wrote:

 If the compiler can tell that B!(int) is a type, why can't it tell that it is
a child class of A!(int) ?


This is a bug. In template specializations, : means equality. In static 
if, : means convertibility.

So, you can use:
template ADefaults(Type, AType)
{
	static assert (is (AType : A!(Type)));
	Type blah()
		{ return Type.init; }
}

http://d.puremagic.com/issues/show_bug.cgi?id=1715 if you want to vote.
Jan 30 2009
parent Christopher Wright <dhasenan gmail.com> writes:
Christopher Wright wrote:

 Mike L. wrote:

 If the compiler can tell that B!(int) is a type, why can't it tell 
 that it is a child class of A!(int) ?


 
 This is a bug. In template specializations, : means equality. In static 
 if, : means convertibility.


This is only with template specializations involving templates, by the 
way. Template specializations involving any other type are fine.
Jan 30 2009