fix quirky negative sign expression markup parsing

lib/liquid.rb

@@ -68,7 +68,6 @@ require 'liquid/extensions'
 require 'liquid/errors'
 require 'liquid/interrupts'
 require 'liquid/strainer_template'
-require 'liquid/expression'
 require 'liquid/context'
 require 'liquid/tag'
 require 'liquid/block_body'
@@ -77,6 +76,7 @@ require 'liquid/variable'
 require 'liquid/variable_lookup'
 require 'liquid/range_lookup'
 require 'liquid/resource_limits'
+require 'liquid/expression'
 require 'liquid/template'
 require 'liquid/condition'
 require 'liquid/utils'

lib/liquid/expression.rb

@@ -57,7 +57,8 @@ module Liquid
       'true' => true,
       'false' => false,
       'blank' => '',
-      'empty' => ''
+      'empty' => '',
+      '-' => VariableLookup.parse("-")
     }.freeze
 
     DOT = ".".ord

test/integration/expression_test.rb

@@ -23,7 +23,7 @@ class ExpressionTest < Minitest::Test
   end
 
   def test_float
-    assert_template_result("-17.42", "{{ -17.42  }}")
+    assert_template_result("-17.42", "{{ -17.42 }}")
     assert_template_result("2.5", "{{ 2.5 }}")
     assert_expression_result(1.5, "1.5")
   end
@@ -42,6 +42,18 @@ class ExpressionTest < Minitest::Test
     )
   end
 
+  def test_quirky_negative_sign_expression_markup
+    result = Expression.parse("-")
+    assert(result.is_a?(VariableLookup))
+    assert_equal("-", result.name)
+
+    # for this template, the expression markup is "-"
+    assert_template_result(
+      "",
+      "{{ - 'theme.css' - }}",
+    )
+  end
+
   private
 
   def assert_expression_result(expect, markup, **assigns)