• Nathan S. (34/34) Dec 08 2020 The following code fails to compile. Is this a compiler error or
• Basile B. (27/29) Dec 08 2020 What is wrong is that partial specialization is not correct.
• Nathan S. (15/44) Dec 08 2020 Thanks a lot! In my case what I was intending was:

Nathan S. <no.public.email example.com> writes:

The following code fails to compile. Is this a compiler error or 
if not what is wrong with the code?

---
struct Template2(T)
{
     // If both of the following are removed compilation succeeds
     // without any other changes:
     enum tString = T.stringof;
     static if (is(T == class))
         enum tLinkage = __traits(getLinkage, T);
}

struct Template1(Param1, Param2 = Template2!Param1) {}

// Moving the definition of AliasTemplate1S after the definition 
of S
// causes compilation to succeed without any other changes.
alias AliasTemplate1S = Template1!S;

class S
{
     // If the following line is removed compilation succeeds
     // without any other changes.
     Template1!int x;
}

void main()
{
}
---

Failure message:
main.d(20): Error: struct main.Template1(Param1, Param2 = 
Template2!Param1) recursive template expansion
main.d(20):        while looking for match for Template1!int
main.d(7): Error: class S is forward referenced
main.d(10): Error: template instance main.Template2!(S) error 
instantiating
main.d(14):        instantiated from here: Template1!(S)

Basile B. <b2.temp gmx.com> writes:

On Tuesday, 8 December 2020 at 20:11:40 UTC, Nathan S. wrote:
 The following code fails to compile. Is this a compiler error 
 or if not what is wrong with the code?

What is wrong is that partial specialization is not correct.
The correct partial specialization is:

---
struct Template2(T)
{
     enum tString = T.stringof;
     static if (is(T == class))
         enum tLinkage = __traits(getLinkage, T);
}

struct Template1(Param1, Param2 = Template2!Param1) {}

alias AliasTemplate1S(SecondParam) = Template1!(S,SecondParam);
//                    ^here

class S
{
     Template1!int x;
}
---

Now that being said, the compiler could complain about the 
incorrect partial spec instead of falling in the nonsensical 
error message.

There is a gap because the second template param looks optional 
so you dont put it the partial specialization but it is still 
required.

Anyway. This case is either a "bad diagnostic" or an 
"enhancement" request.
Or should be specified.

Nathan S. <no.public.email example.com> writes:

On Tuesday, 8 December 2020 at 22:01:52 UTC, Basile B. wrote:
 On Tuesday, 8 December 2020 at 20:11:40 UTC, Nathan S. wrote:
 The following code fails to compile. Is this a compiler error 
 or if not what is wrong with the code?

 What is wrong is that partial specialization is not correct.
 The correct partial specialization is:

 ---
 struct Template2(T)
 {
     enum tString = T.stringof;
     static if (is(T == class))
         enum tLinkage = __traits(getLinkage, T);
 }

 struct Template1(Param1, Param2 = Template2!Param1) {}

 alias AliasTemplate1S(SecondParam) = Template1!(S,SecondParam);
 //                    ^here

 class S
 {
     Template1!int x;
 }
 ---

 Now that being said, the compiler could complain about the 
 incorrect partial spec instead of falling in the nonsensical 
 error message.

 There is a gap because the second template param looks optional 
 so you dont put it the partial specialization but it is still 
 required.

 Anyway. This case is either a "bad diagnostic" or an 
 "enhancement" request.
 Or should be specified.

Thanks a lot! In my case what I was intending was:

alias AliasTemplate1S = Template1!(S, Template2!S)

which as you suggest works fine. It's a bit odd that the 
non-optional second parameter becomes optional again if 
declaration order is shuffled, but my motivation to look into 
this has temporarily abated since it's no longer stopping me from 
doing something else.

For the program where I ran into this problem the most convenient 
fix turns out to be to get rid of the default template parameter 
and instead use a pattern like this:

---
struct Template1(Param1, Param2) {}
alias Template1(Param1) = Template1!(Param1, Template2!Param1);
---