Make index redirect warning case-sensitive (#1077)

lib/ex_doc/formatter/html.ex

@@ -375,14 +375,23 @@ defmodule ExDoc.Formatter.HTML do
   end
 
   defp generate_redirect(filename, config, redirect_to) do
-    unless File.regular?("#{config.output}/#{redirect_to}") do
+    unless case_sensitive_file_regular?("#{config.output}/#{redirect_to}") do
       IO.puts(:stderr, "warning: #{filename} redirects to #{redirect_to}, which does not exist")
     end
 
     content = Templates.redirect_template(config, redirect_to)
     File.write!("#{config.output}/#{filename}", content)
   end
 
+  defp case_sensitive_file_regular?(path) do
+    if File.regular?(path) do
+      files = path |> Path.dirname() |> File.ls!()
+      Path.basename(path) in files
+    else
+      false
+    end
+  end
+
   def filter_list(:module, nodes) do
     Enum.filter(nodes, &(not (&1.type in [:exception, :task])))
   end

test/ex_doc/formatter/html_test.exs

@@ -100,12 +100,12 @@ defmodule ExDoc.Formatter.HTMLTest do
   test "warns when generating an index.html file with an invalid redirect" do
     output =
       capture_io(:stderr, fn ->
-        generate_docs(doc_config(main: "Unknown"))
+        generate_docs(doc_config(main: "Randomerror"))
       end)
 
-    assert output == "warning: index.html redirects to Unknown.html, which does not exist\n"
+    assert output == "warning: index.html redirects to Randomerror.html, which does not exist\n"
     assert File.regular?("#{output_dir()}/index.html")
-    refute File.regular?("#{output_dir()}/Unknown.html")
+    assert File.regular?("#{output_dir()}/RandomError.html")
   end
 
   test "warns on undefined functions" do