Templates with alias param instantiated with null cached by DMD..?

[Kenji Hara]

On Wednesday, 23 October 2013 at 07:22:56 UTC, simendsjo wrote:
> I've stumbled upon a strange bug, and I'm not sure what I 
> should write in the bug report. Could someone explain what's 
> going on here or file the bug for me?
>
> template A(alias T) {
>     alias A = T;
> }
>
> void main() {
>     struct S1 { S1* p; }
>     static assert(is(typeof(A!(S1.init.p)) == S1*)); // ok
>
>     pragma(msg, "NULL: ", typeof(A!(null))); // fail: S1*
>
>     struct S2 { S2* p; }
>     static assert(is(typeof(A!(S2.init.p)) == S2*)); // fail: 
> S1*
> }

A!(S1.init.p) is mostly same as A!( cast(S1*)null ), because the 
expression S1.init.p is interpreted to a null value.

1. But current ABI does not support "a typed null template value 
argument" because all of null expression on template argument are 
mangled to 'n'. So the type of null value will be never encoded 
in the mangled name, and the three instantiations A!(S1.init.p), 
A!(null) and A!(S2.init.p) will have exactly same mangling.

2. However, the first instantiation A!(S1.init.p) wrongly caches 
the null value type (== S1*), and it appears in following 
instantiations during semantic analysis phase.

#2 is definitely a front-end bug. However I'm not sure the 
current ABI definition issue (== #2) is a language spec bug or 
not...

Kenji Hara