fix #20645 - implement C++ style init-statement for if

[thewilsonator]

Sill to do

• same thing but for switch

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[thewilsonator]

Silly question: why does the old code do parseAssignCondition followed by parseExpression? Does parseAssignCondition not eat tokens?

Unlike with for which (assuming its there) unconditionally parses a statement before the first semicolon, I think here we would need to determine if there is a semicolon present before deciding wether or not to sparse a statement or an expression here.

[tgehr]

parseAssignCondition only eats tokens if it sits on a storage class. If it does not, it just returns null. It also does not parse an initializer.

AFAIU, in if(auto x = e), parseAssignCondition parses the auto x = part, and parseExpression parses the e part.

[Lars-Kristiansen]

Hello I'm baz from styx-lang. What you're doing is bad. What you need is "variable declaration" as a primary expression, but you guys keep on making special cases every each time Manu is upset.