[parser] a => counter+=1 parsed as (a => counter) += 1, violating the spec, and causing issues with =>

[timotheecour]

/cc @Araq

import macros

dumpTree:
  # violates spec: arrow like operators have lowest precedence
  a1 => counter += 2
  # ditto
  a2 => counter *= 2
  # violates spec: `^` should have higher precedence than += (regardless of `^` being right-associative)
  a3 ^= counter += 2

StmtList
  Infix
    Ident "+="
    Infix
      Ident "=>"
      Ident "a1"
      Ident "counter"
    IntLit 2
  Infix
    Ident "*="
    Infix
      Ident "=>"
      Ident "a2"
      Ident "counter"
    IntLit 2
  Infix
    Ident "^="
    Ident "a3"
    Infix
      Ident "+="
      Ident "counter"
      IntLit 2

NOTE

this affects sugar.nim =>, zero-functional, and lambda PR #8679

spec:

https://github.com/nim-lang/Nim/blob/devel/doc/manual.rst#L573
/Users/timothee/git_clone/nim/Nim/doc/manual.rst

[metagn]

1 here has to be 0. I would almost hope it's not a typo because it's been there since the initial implementation of arrow operators (2014).

Nim/compiler/lexer.nim

Line 976 in af14096

tok.ident.s[^2] in {'-', '~', '='}: return 1