Fix whereami to work after a chdir in a main script

fixes #1220

lib/pry/commands/whereami.rb

@@ -44,11 +44,31 @@ class << self
       def setup
         if target.respond_to?(:source_location)
           file, @line = target.source_location
-          @file = expand_path(file)
         else
-          @file = expand_path(target.eval('__FILE__'))
+          file = target.eval('__FILE__')
           @line = target.eval('__LINE__')
         end
+
+        # If the current location is in a top-level script, then the filename
+        # returned by source_location or __FILE__ will be just the filename with
+        # no path: e.g. 'myscript.rb'. If the current working directory has been
+        # changed, then expand_path(file) will construct an incorrect path to
+        # the source. We can use __dir__ to fix this case.
+        if !file.nil? && !File.absolute_path?(file)
+          dir = target.eval('__dir__')
+
+          # We have to be careful not to join with dir if it's also a relative
+          # path. There are some cases where the file and dir paths are both
+          # relative to the same directory, like:
+          #
+          # __FILE__: "spec/fixtures/example.erb"
+          # __dir__:  "spec/fixtures"
+          if !dir.nil? && File.absolute_path?(dir)
+            file = File.join(dir, file)
+          end
+        end
+
+        @file = expand_path(file)
         @method = Pry::Method.from_binding(target)
       end
 

spec/commands/whereami_spec.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'method_source'
+require 'open3'
 
 describe "whereami" do
   it 'should work with methods that have been undefined' do
@@ -277,4 +278,14 @@ def blimey!
   it "should work inside an object" do
     expect(pry_eval(Object.new, 'whereami')).to match(/Inside #<Object/)
   end
+
+  it 'should work after chdir from main script' do
+    script_dir = File.expand_path(File.join(__dir__, '..', 'fixtures'))
+    script = 'whereami_main_chdir.rb'
+
+    output, status = Open3.capture2(RbConfig.ruby, script, chdir: script_dir)
+
+    expect(output.strip).to eq File.join(script_dir, script)
+  end
+
 end

spec/fixtures/whereami_main_chdir.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+if $PROGRAM_NAME != __FILE__
+  fail "#{__FILE__} should only be executed as a top level program"
+end
+
+$LOAD_PATH.unshift File.expand_path(File.join(__dir__, '..', '..', 'lib'))
+
+require 'pry'
+require 'pry/testable'
+
+include Pry::Testable::Evalable
+
+class Cor
+  def blimey!
+    Dir.chdir '..' do
+      pry_eval(binding, 'whereami', '_file_')
+    end
+  end
+end
+
+puts Cor.new.blimey!