Fix up did_you_mean on older ruby versions

Author: kddnewton

lib/thor/error.rb

@@ -1,4 +1,11 @@
 class Thor
+  Correctable =
+    begin
+      require 'did_you_mean'
+      DidYouMean::Correctable
+    rescue LoadError
+    end
+
   # Thor::Error is raised when it's caused by wrong usage of thor classes. Those
   # errors have their backtrace suppressed and are nicely shown to the user.
   #
@@ -38,7 +45,7 @@ def initialize(command, all_commands, namespace)
       super(message)
     end
 
-    prepend DidYouMean::Correctable
+    prepend Correctable if Correctable
   end
   UndefinedTaskError = UndefinedCommandError
 
@@ -78,7 +85,7 @@ def initialize(switches, unknown)
       super("Unknown switches #{unknown.map(&:inspect).join(', ')}")
     end
 
-    prepend DidYouMean::Correctable
+    prepend Correctable if Correctable
   end
 
   class RequiredArgumentMissingError < InvocationError
@@ -87,8 +94,10 @@ class RequiredArgumentMissingError < InvocationError
   class MalformattedArgumentError < InvocationError
   end
 
-  DidYouMean::SPELL_CHECKERS.merge!(
-    'Thor::UndefinedCommandError' => UndefinedCommandError::SpellChecker,
-    'Thor::UnknownArgumentError' => UnknownArgumentError::SpellChecker
-  )
+  if Correctable
+    DidYouMean::SPELL_CHECKERS.merge!(
+      'Thor::UndefinedCommandError' => UndefinedCommandError::SpellChecker,
+      'Thor::UnknownArgumentError' => UnknownArgumentError::SpellChecker
+    )
+  end
 end

spec/base_spec.rb

@@ -263,10 +263,8 @@ def hello
     end
 
     it "suggests commands that are similar if there is a typo" do
-      expected = <<~MSG
-        Could not find command "paintz" in "barn" namespace.
-        Did you mean?  "paint"
-      MSG
+      expected = "Could not find command \"paintz\" in \"barn\" namespace.\n"
+      expected << "Did you mean?  \"paint\"" if Thor::Correctable
 
       expect(capture(:stderr) { Barn.start(%w(paintz)) }).to eq(expected)
     end

spec/parser/options_spec.rb

@@ -114,10 +114,8 @@ def remaining
       create :foo => "baz", :bar => :required
       parse("--bar", "baz", "--baz", "unknown")
 
-      expected = <<~MSG.chomp
-        Unknown switches "--baz"
-        Did you mean?  "--bar"
-      MSG
+      expected = "Unknown switches \"--baz\""
+      expected << "\nDid you mean?  \"--bar\"" if Thor::Correctable
 
       expect { check_unknown! }.to raise_error(Thor::UnknownArgumentError, expected)
     end