[Issue 18393] REG(v2.078.1=>master) function re-declarations causes conflict, but should not

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18393

--- Comment #2 from Timothee Cour <timothee.cour2 at gmail.com> ---
on a vaguely related note:

```
void fun_bad3(T)(T a);
void fun_bad3(T)(T a){}

void test(){
  fun_bad3(1);
}
```

Error: test_all.fun_bad3 called with argument types (int) matches both:
main.d(11):     test_all.fun_bad3!int.fun_bad3(int a)
and:
main.d(12):     test_all.fun_bad3!int.fun_bad3(int a)

is that intended that only functions can be pre-declared, but not function
templates?

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