To get the "0+0" matching case rejected again I had to flesh out a bit more of the AST validator. This initial piece of the validator replaces the previously pattern-matching-specific AST optimisation code that refused to fold BinOp expressions that weren't complex numbers, allowing the compiler to reject them.

I also added two new tests (240b and 241b) to ensure that 0+0 & f"" were also rejected as mapping keys. 241b passes with the current PR, as f"" gets rejected by both the check in the AST validator *and* by the "constant or attribute lookup" restriction in the compiler, so the latter check covers for the leaky validator.

240b is disabled for now, as it won't pass until the AST validator checks all the subexpressions being used as keys in a mapping pattern (because it gets constant folded to "0", the check in the compiler accepts it).

That leaves fixing the unparser tests as the key thing to do before I create the main PR.

Before that, though, I'll look at potentially simplifying the MatchMapping code by changing the signature as Brandt suggested.