[Issue 23986] New: ICE: dip1021 asserts on `typeof(null)` parameter

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23986

          Issue ID: 23986
           Summary: ICE: dip1021 asserts on `typeof(null)` parameter
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

```
// REQUIRED_ARGS: -preview=dip1021 -o-
@safe:

void f(typeof(null) obj, int* x) {}

void g()
{
    f(null, null);
}
```

Hits the `default: assert(0)` in dmd.escape.isReferenceToMutable, because it
doesn't have a case for `Tnull`.

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