Ensure old manifests can be upgraded

Author: José Valim

lib/mix/lib/mix/compilers/elixir.ex

@@ -488,12 +488,14 @@ defmodule Mix.Compilers.Elixir do
     try do
       manifest |> File.read!() |> :erlang.binary_to_term()
     rescue
-      _ -> {[], []}
+      _ ->
+        delete_old_manifest(manifest, compile_path)
+        {[], []}
     else
       [@manifest_vsn | data] ->
         split_manifest(data, compile_path)
 
-      [v | data] when v in [:v4, :v5, :v6, :v7, :v8] ->
+      [v | data] when is_integer(v) ->
         for module <- data,
             is_record(module, :module),
             do: File.rm(Path.join(compile_path, module(module, :beam)))
@@ -505,6 +507,26 @@ defmodule Mix.Compilers.Elixir do
     end
   end
 
+  defp delete_old_manifest(manifest, compile_path) do
+    manifest = Path.expand("../../.compile.elixir", manifest)
+
+    try do
+      manifest
+      |> File.read!()
+      |> :erlang.binary_to_term()
+      |> case do
+        [v | data] when v in [:v4, :v5, :v6, :v7, :v8] ->
+          for module <- data,
+              is_record(module, :module),
+              do: File.rm(Path.join(compile_path, module(module, :beam)))
+      end
+
+      File.rm(manifest)
+    rescue
+      _ -> :ok
+    end
+  end
+
   defp split_manifest(data, compile_path) do
     Enum.reduce(data, {[], []}, fn
       module() = module, {modules, sources} ->