Block ampersand association

Fix parsing expressions like `foo.instance_exec(&T.must(block))`, where there are two `args_add_block` calls with a single `&`. Previously it was associating the `&` with the wrong block.

Author: kddnewton

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 
 ## [Unreleased]
 
+### Changed
+
+- [#45](https://github.com/ruby-syntax-tree/syntax_tree/issues/45) - Fix parsing expressions like `foo.instance_exec(&T.must(block))`, where there are two `args_add_block` calls with a single `&`. Previously it was associating the `&` with the wrong block.
+
 ## [2.1.0] - 2022-04-12
 
 ### Added

lib/syntax_tree/parser.rb

@@ -414,12 +414,19 @@ def on_args_add(arguments, argument)
     #     (false | untyped) block
     #   ) -> Args
     def on_args_add_block(arguments, block)
+      # First, see if there is an & operator that could potentially be
+      # associated with the block part of this args_add_block. If there is not,
+      # then just return the arguments.
       operator = find_token(Op, "&", consume: false)
-
-      # If we can't find the & operator, then there's no block to add to the
-      # list, so we're just going to return the arguments as-is.
       return arguments unless operator
 
+      # If there are any arguments and the operator we found from the list is
+      # not after them, then we're going to return the arguments as-is because
+      # we're looking at an & that occurs before the arguments are done.
+      if arguments.parts.any? && operator.location.start_char < arguments.location.end_char
+        return arguments
+      end
+
       # Now we know we have an & operator, so we're going to delete it from the
       # list of tokens to make sure it doesn't get confused with anything else.
       tokens.delete(operator)
@@ -428,13 +435,6 @@ def on_args_add_block(arguments, block)
       location = operator.location
       location = operator.location.to(block.location) if block
 
-      # If there are any arguments and the operator we found from the list is
-      # not after them, then we're going to return the arguments as-is because
-      # we're looking at an & that occurs before the arguments are done.
-      if arguments.parts.any? && location.start_char < arguments.location.end_char
-        return arguments
-      end
-
       # Otherwise, we're looking at an actual block argument (with or without a
       # block, which could be missing because it could be a bare & since 3.1.0).
       arg_block = ArgBlock.new(value: block, location: location)

test/fixtures/arg_block.rb

@@ -18,3 +18,5 @@
 def foo(&)
   bar(&)
 end
+% # https://github.com/ruby-syntax-tree/syntax_tree/issues/45
+foo.instance_exec(&T.must(block))