use arrow precedence in spec (#20166)

* test using arrow precedence in spec

refs #8759

* add test for #8759

compiler/lexer.nim

@@ -973,7 +973,7 @@ proc getPrecedence*(tok: Token): int =
 
     # arrow like?
     if tok.ident.s.len > 1 and tok.ident.s[^1] == '>' and
-      tok.ident.s[^2] in {'-', '~', '='}: return 1
+      tok.ident.s[^2] in {'-', '~', '='}: return 0
 
     template considerAsgn(value: untyped) =
       result = if tok.ident.s[^1] == '=': 1 else: value

tests/parser/tprecedence.nim

@@ -54,3 +54,10 @@ let
 const
   test =
     proc(): int = 1
+
+# bug #8759
+block:
+  template `=>`(a, b): untyped = (a, b)
+  template `+=`(a, b): untyped = a * b
+
+  doAssert ("abc" => 3 += 5) == ("abc", 15)