[Issue 22209] New: NRVO variable detection ignoring `alias this` conversion => segfaults

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22209

          Issue ID: 22209
           Summary: NRVO variable detection ignoring `alias this`
                    conversion => segfaults
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: kinke at gmx.net

This segfaults at runtime (for all versions currently on run.dlang.io):
```
struct U { size_t[4] a; }

struct S
{
    int x;
    U u;
    alias u this;
}

U foo()
{
    S s = S(42, U([1, 2, 3, 4]));
    return s;
}

void main()
{
    assert(foo().a[0] == 1);
}
```

It's apparently caused by the frontend wrongly marking `s` in `foo()` as the
NRVO variable (FuncDeclaration.nrvo_var), ignoring that it requires an implicit
conversion via `alias this` - return type and variable type clearly differ.

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