[Issue 3072] tuples can't be aliases ( except when the aliases are templates. )

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=3072

--- Comment #3 from Kenji Hara <k.hara.pg at gmail.com> ---
(In reply to Rob Jacques from comment #1)
> In general template tuples can't contain alias template parameters but it
> appears that there is an exception to this rule when the parameter is itself
> a template. 
> 
> DMD 2.049
> 
> T foo(T)(T t) { return t; }
> template map1(fun...) {
>     enum map1 = 42;
> }
> template map2(T, U...) {
>     static if(U.length > 0) enum map2 = map2!(U);
>     else                    enum map2 = 42;
> }
> 
> void main() {
>     auto x = map1!(char,foo);  // compiles
>     auto y = map2!(char,foo);  // doesn't compile
>     auto z = map1!(char,map1); // compiles
>     auto w = map2!(char,map1); // doesn't compile
>     return;
> }
> 
> I've added the accepts-invalid, rejects-valid and spec keywords, since
> tuples of aliases should either work or not-work and the behavior should be
> documented in the spec. I'd lean towards not-work, since tuples of aliases
> seem to be buggy (i.e. Issue 5087) not to mention that alias parameters are
> buggy(i.e. issue 5082)

This test case is invalid. map2 is recursively instantiated, but in the line:

>     static if(U.length > 0) enum map2 = map2!(U);

the instantiation map2!(foo) or map2!(map1) fails because any template symbols
cannot match to the type parameter T.

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