[Issue 20187] New: alias this bypasses truncating conversion warning

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20187

          Issue ID: 20187
           Summary: alias this bypasses truncating conversion warning
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

Compile with -w:
```
unittest {
    struct X {
        long y;
        alias y this;
    }
    X x;
    x -= 1.0; //No warning
    x.y -= 1.0; //Warning
}
```
Only one warning is triggered, when two should be.

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