diff --git a/lib/mix/lib/mix/compilers/elixir.ex b/lib/mix/lib/mix/compilers/elixir.ex
index bd9e178231b..4a6ba45e0c8 100644
--- a/lib/mix/lib/mix/compilers/elixir.ex
+++ b/lib/mix/lib/mix/compilers/elixir.ex
@@ -138,8 +138,6 @@ defmodule Mix.Compilers.Elixir do
     {sources, removed_modules} =
       update_stale_sources(sources, stale, removed_modules, sources_stats)
 
-    if opts[:all_warnings], do: show_warnings(sources)
-
     if stale != [] do
       Mix.Utils.compiling_n(length(stale), :ex)
       Mix.Project.ensure_structure()
@@ -157,6 +155,12 @@ defmodule Mix.Compilers.Elixir do
       else
         {:ok, _, warnings} ->
           {modules, _exports, sources, pending_modules, _pending_exports} = get_compiler_info()
+
+          # We only collect the warnings if --all-warnings is given.
+          # In this case, we print them too. Then we apply the new warnings.
+          previous_warnings =
+            if opts[:all_warnings], do: previous_warnings(sources, true), else: []
+
           sources = apply_warnings(sources, warnings)
 
           write_manifest(
@@ -172,14 +176,16 @@ defmodule Mix.Compilers.Elixir do
           )
 
           put_compile_env(sources)
-          {:ok, Enum.map(warnings, &diagnostic(&1, :warning))}
+          all_warnings = previous_warnings ++ Enum.map(warnings, &diagnostic(&1, :warning))
+          unless_previous_warnings_as_errors(previous_warnings, opts, {:ok, all_warnings})
 
         {:error, errors, warnings} ->
-          # In case of errors, we show all previous warnings and all new ones
+          # In case of errors, we show all previous warnings and all new ones.
+          # Print the new ones if --all-warnings was given.
           {_, _, sources, _, _} = get_compiler_info()
           errors = Enum.map(errors, &diagnostic(&1, :error))
           warnings = Enum.map(warnings, &diagnostic(&1, :warning))
-          {:error, warning_diagnostics(sources) ++ warnings ++ errors}
+          {:error, previous_warnings(sources, opts[:all_warnings]) ++ warnings ++ errors}
       after
         Code.compiler_options(previous_opts)
         Mix.Compilers.ApplicationTracer.stop(pending_tracer)
@@ -217,7 +223,8 @@ defmodule Mix.Compilers.Elixir do
         )
       end
 
-      {status, warning_diagnostics(sources)}
+      previous_warnings = previous_warnings(sources, opts[:all_warnings])
+      unless_previous_warnings_as_errors(previous_warnings, opts, {status, previous_warnings})
     end
   after
     Mix.Compilers.ApplicationTracer.stop()
@@ -803,13 +810,13 @@ defmodule Mix.Compilers.Elixir do
     _ = :code.delete(module)
   end
 
-  defp show_warnings(sources) do
-    for source(source: source, warnings: warnings) <- sources do
-      file = Path.absname(source)
-
-      for {location, message} <- warnings do
-        Kernel.ParallelCompiler.print_warning({file, location, message})
-      end
+  defp previous_warnings(sources, print?) do
+    for source(source: source, warnings: warnings) <- sources,
+        file = Path.absname(source),
+        {location, message} <- warnings do
+      warning = {file, location, message}
+      print? && Kernel.ParallelCompiler.print_warning(warning)
+      diagnostic(warning, :warning)
     end
   end
 
@@ -821,12 +828,6 @@ defmodule Mix.Compilers.Elixir do
     end
   end
 
-  defp warning_diagnostics(sources) do
-    for source(source: source, warnings: warnings) <- sources,
-        {location, message} <- warnings,
-        do: diagnostic({Path.absname(source), location, message}, :warning)
-  end
-
   defp diagnostic({file, location, message}, severity) do
     %Mix.Task.Compiler.Diagnostic{
       file: file,
@@ -1014,4 +1015,14 @@ defmodule Mix.Compilers.Elixir do
   defp delete_checkpoint(manifest) do
     File.rm(manifest <> ".checkpoint")
   end
+
+  defp unless_previous_warnings_as_errors(previous_warnings, opts, {status, all_warnings}) do
+    if previous_warnings != [] and opts[:warnings_as_errors] do
+      message = "Compilation failed due to warnings while using the --warnings-as-errors option"
+      IO.puts(:stderr, message)
+      {:error, all_warnings}
+    else
+      {status, all_warnings}
+    end
+  end
 end
diff --git a/lib/mix/test/mix/tasks/compile.elixir_test.exs b/lib/mix/test/mix/tasks/compile.elixir_test.exs
index b2958451991..0090fc4a75d 100644
--- a/lib/mix/test/mix/tasks/compile.elixir_test.exs
+++ b/lib/mix/test/mix/tasks/compile.elixir_test.exs
@@ -1267,6 +1267,41 @@ defmodule Mix.Tasks.Compile.ElixirTest do
     end)
   end
 
+  test "warning from --all-warnings are treated as errors with --warnings-as-errors" do
+    in_fixture("no_mixfile", fn ->
+      Mix.Project.push(MixTest.Case.Sample)
+
+      File.write!("lib/a.ex", """
+      defmodule A do
+        def my_fn(unused), do: :ok
+      end
+      """)
+
+      message = "variable \"unused\" is unused"
+
+      assert capture_io(:stderr, fn ->
+               Mix.Tasks.Compile.Elixir.run([]) == :ok
+             end) =~ message
+
+      # Stale compilation fails due to warnings as errors
+      assert capture_io(:stderr, fn ->
+               catch_exit(Mix.Task.run("compile", ["--all-warnings", "--warnings-as-errors"]))
+             end) =~ message
+
+      Mix.Task.clear()
+
+      File.write!("lib/b.ex", """
+      defmodule B do
+      end
+      """)
+
+      # Recompiling a new file still fails due to warnings as errors
+      assert capture_io(:stderr, fn ->
+               catch_exit(Mix.Task.run("compile", ["--all-warnings", "--warnings-as-errors"]))
+             end) =~ message
+    end)
+  end
+
   test "returns warning diagnostics" do
     in_fixture("no_mixfile", fn ->
       Mix.Project.push(MixTest.Case.Sample)