Weird precedence of unary operators, `where` and syntactic unaries

[c42f]

The precedence of type operators <: is meant to be lower than where as handled with a special rule parse_unary_subtype (see https://github.com/JuliaLang/julia/pull/21567).

But the way this is implemented in the reference parser (and copied here) the precedence doesn't work when chained with normal unary ops. This happens because parse_unary is still involved in parsing chained unary type operators.

julia> parsestmt(SyntaxNode, "<: A where B")
line:col│ tree                                   │ file_name
   1:1  │[<:-pre]
   1:3  │  [where]
   1:4  │    A
   1:12 │    B

julia> parsestmt(SyntaxNode, "+ <: A where B")
line:col│ tree                                   │ file_name
   1:1  │[where]
   1:1  │  [call-pre]
   1:1  │    +
   1:2  │    [<:-pre]
   1:6  │      A
   1:14 │  B

Fixing this isn't super simple - if we call parse_unary_subtype from within parse_unary, we'll break the precedence of juxtaposition which is interposed between these two. For example, we must have "√2i" parse as (juxtapose (call-pre √ 2) i).

[c42f]

@JeffBezanson do you think there's a way out of this conundrum?