what would it break? It litterally was proposed by an extension, with support from Stroustrup.
IIRC Stroustrup regrets using . for member function calls (as opposed to just some syntactic sugar for any function call)
The way it was worded in the GP comment would change name lookup in existing well-formed programs, making them fail to compile, or compile differently.
I'm not familiar with Stroustrup's proposal, but I doubt that he proposed it without some opt-in annotation.
It really couldn't. It would be a breaking change.