[Issue 21255] "overload alias ... forward declaration" when overload set of imported template functions is passed to alias template parameter

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21255

--- Comment #1 from FeepingCreature <default_357-line at yahoo.de> ---
Simpler repro:

```
void decodeA()() { }
void decodeB()() { }

alias decode = decodeA;
alias decode = decodeB;

void foo(alias A)() { }

void main() { foo!decode; }
```

Seems it doesn't know how to mangle overload sets created by alias.

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