[Issue 21288] Wrong context pointer for alias this function

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21288

--- Comment #2 from Richard Manthorpe <rmanth at gmail.com> ---
It feels like it should be valid. If there is an `alias this` you should really
have to know whether something is in `B` or `A`. Since you can do things like
`A.p.stringof` or `__traits(getAttributes, A.p)` which requires a symbol, you
should probably also be able to do `B.p.stringof` or `__traits(getAttributes,
B.p)`.

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