[Issue 23686] New: template instance reused with default alas arg

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23686

          Issue ID: 23686
           Summary: template instance reused with default alas arg
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: accepts-invalid
          Severity: critical
          Priority: P2
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: destructionator at gmail.com

```
void FNfoo() {}
void load(string name, alias T = mixin("FN" ~ name))() {}

void main() {
        load!"foo"();
        load!"wtf"();
}
```

Compiles without error even though FNwtf does not exist.


Smells similar to https://issues.dlang.org/show_bug.cgi?id=12533 and
https://issues.dlang.org/show_bug.cgi?id=14501

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