[Issue 21674] New: [REG v2.086] `alias this` triggers wrong deprecation message on function call

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21674

          Issue ID: 21674
           Summary: [REG v2.086] `alias this` triggers wrong deprecation
                    message on function call
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: rejects-valid
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

Originally posted as a comment on the PR that introduced the regression:
https://github.com/dlang/dmd/pull/9323#issuecomment-686852776

```
struct Module
{
    CachedString data;
}

struct CachedString
{
    private size_t len;

    this (string data) { this.len = data.length; }
    public string str () const { return null; }
    public void str (string value) { this.len = value.length; assert(0); }

    alias str this;
}

void main ()
{
    Module m;
    m.data = "Hello World";
}
```

I added an `assert(0);` to confirm that `str` is being called (and it is).
This triggers the following deprecation message:
```
dep.d(20): Deprecation: Cannot use `alias this` to partially initialize
variable `m.data` of type `CachedString`. Use `m.data.str`
```

This message is wrong, the `str` setter gets called (as expected).

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