[Issue 11986] IFTI type deduction / implicit conversion does not work for null literal

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=11986

--- Comment #2 from basile-z <b2.temp at gmx.com> ---
The bug is much more specific than suggested by the summary.

1. it is limited to classes because the following works

```
struct S(T){}

void v(T)(S!T* a, S!T* b) {}

void main()
{
    S!int* a;
    v(a,null);
}   
```

2. it limited to classes templates because the following works

```
class S{}

void v(T)(T a, T b) {}

void main()
{
    S a;
    v(a,null);
}  
```

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