[yencabulator]

Parent means

    var bestShape atomic.Value
    bestShape.Store((*Circle)(nil))

[morelisp]

Which will store it as a *Circle, and only allow more *Circles, not Shapes. That part of GP’s claim is correct.

It just had nothing to do with atomicity; it means something specific, not just “I like the failure mode.”

[yencabulator]

That's pretty easy to workaround:

    type shapeContainer struct { Shape }

The usual way to use atomic.Value is by writing strongly-typed wrappers anyway, so that doesn't affect your codebase beyond about 3 lines.