Ensure proper precedence between & and operators, closes #7412

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/elixir/lib/code/formatter.ex

@@ -10,6 +10,7 @@ defmodule Code.Formatter do
   @min_line 0
   @max_line 9_999_999
   @empty empty()
+  @ampersand_prec Code.Identifier.unary_op(:&) |> elem(1)
 
   # Operators that do not have space between operands
   @no_space_binary_operators [:..]
@@ -788,10 +789,11 @@ defmodule Code.Formatter do
   end
 
   defp binary_operand_to_algebra(operand, context, state, parent_op, parent_info, side, nesting) do
+    {parent_assoc, parent_prec} = parent_info
+
     with {op, meta, [left, right]} <- operand,
          op_info = Code.Identifier.binary_op(op),
          {_assoc, prec} <- op_info do
-      {parent_assoc, parent_prec} = parent_info
       op_string = Atom.to_string(op)
 
       cond do
@@ -817,7 +819,9 @@ defmodule Code.Formatter do
           binary_op_to_algebra(op, op_string, meta, left, right, context, state, 2)
       end
     else
-      {:&, _, [arg]} when not is_integer(arg) and side == :left ->
+      {:&, _, [arg]}
+      when not is_integer(arg) and side == :left
+      when not is_integer(arg) and parent_assoc == :left and parent_prec > @ampersand_prec ->
         {doc, state} = quoted_to_algebra(operand, context, state)
         {wrap_in_parens(doc), state}
 

lib/elixir/test/elixir/code_formatter/integration_test.exs

@@ -453,4 +453,50 @@ defmodule Code.Formatter.IntegrationTest do
             | :other
     """
   end
+
+  test "capture with operators" do
+    assert_same """
+    "this works" |> (&String.upcase/1) |> (&String.downcase/1)
+    """
+
+    assert_same """
+    "this works" || (&String.upcase/1) || (&String.downcase/1)
+    """
+
+    assert_same """
+    "this works" == (&String.upcase/1) == (&String.downcase/1)
+    """
+
+    bad = """
+    "this works" = (&String.upcase/1) = (&String.downcase/1)
+    """
+
+    assert_format bad, """
+    "this works" = (&String.upcase/1) = &String.downcase/1
+    """
+
+    bad = """
+    "this works" ++ (&String.upcase/1) ++ (&String.downcase/1)
+    """
+
+    assert_format bad, """
+    "this works" ++ (&String.upcase/1) ++ &String.downcase/1
+    """
+
+    bad = """
+    "this works" | (&String.upcase/1) | (&String.downcase/1)
+    """
+
+    assert_format bad, """
+    "this works" | (&String.upcase/1) | &String.downcase/1
+    """
+
+    bad = ~S"""
+    "this works" \\ (&String.upcase/1) \\ (&String.downcase/1)
+    """
+
+    assert_format bad, ~S"""
+    "this works" \\ &String.upcase/1 \\ &String.downcase/1
+    """
+  end
 end

lib/elixir/test/elixir/code_formatter/operators_test.exs

@@ -812,9 +812,9 @@ defmodule Code.Formatter.OperatorsTest do
     end
 
     test "with operators outside" do
-      assert_same "(& &1) == & &2"
-      assert_same "(& &1) and & &2"
-      assert_same "(&foo/1) and &bar/1"
+      assert_same "(& &1) == (& &2)"
+      assert_same "(& &1) and (& &2)"
+      assert_same "(&foo/1) and (&bar/1)"
       assert_same "[(&IO.puts/1) | &IO.puts/2]"
     end
 

lib/elixir/test/elixir/kernel/fn_test.exs

@@ -58,7 +58,7 @@ defmodule Kernel.FnTest do
     assert (&List.flatten/1).([[0]]) == [0]
     assert (&List.flatten/1).([[0]]) == [0]
     assert (&List.flatten(&1)).([[0]]) == [0]
-    assert (&List.flatten(&1)) == &List.flatten/1
+    assert (&List.flatten(&1)) == (&List.flatten/1)
   end
 
   test "capture local" do
@@ -77,7 +77,7 @@ defmodule Kernel.FnTest do
     assert (&is_atom/1).(:a)
     assert (&is_atom/1).(:a)
     assert (&is_atom(&1)).(:a)
-    assert (&is_atom(&1)) == &is_atom/1
+    assert (&is_atom(&1)) == (&is_atom/1)
   end
 
   test "capture macro" do
@@ -102,7 +102,7 @@ defmodule Kernel.FnTest do
     mod = List
     assert (&mod.flatten(&1)).([1, [2], 3]) == [1, 2, 3]
     assert (&mod.flatten/1).([1, [2], 3]) == [1, 2, 3]
-    assert (&mod.flatten/1) == &List.flatten/1
+    assert (&mod.flatten/1) == (&List.flatten/1)
   end
 
   test "local partial application" do