Accordingly mark top level optional dependencies from child deps

lib/mix/lib/mix/dep.ex

@@ -117,18 +117,42 @@ defmodule Mix.Dep do
     end
   end
 
+  # optional and runtime only matter at the top level.
+  # Any non-top level dependency that is optional and
+  # is still available means it has been fulfilled.
+  @child_keep_opts [:optional, :runtime]
+
   defp load_and_cache(_config, top, top, env) do
     converge(env: env)
   end
 
   defp load_and_cache(config, _top, bottom, _env) do
     {_, deps} = Mix.ProjectStack.read_cache({:cached_deps, bottom})
     app = Keyword.fetch!(config, :app)
-    top_level = for dep <- deps, dep.app == app, child <- dep.deps, do: child.app
-
     seen = populate_seen(MapSet.new(), [app])
     children = get_deps(deps, tl(Enum.uniq(get_children(deps, seen, [app]))))
-    Enum.map(children, &%{&1 | top_level: &1.app in top_level})
+
+    top_level =
+      for dep <- deps,
+          dep.app == app,
+          child <- dep.deps,
+          do: {child.app, Keyword.take(child.opts, @child_keep_opts)},
+          into: %{}
+
+    Enum.map(children, fn %{app: app} = dep ->
+      case top_level do
+        %{^app => child_opts} ->
+          opts =
+            dep.opts
+            |> Keyword.drop(@child_keep_opts)
+            |> Keyword.merge(child_opts)
+
+          %{dep | top_level: true, opts: opts}
+
+        %{} ->
+          %{dep | top_level: false}
+      end
+    end)
   end
 
   defp read_cached_deps(project, env) do

lib/mix/test/mix/dep_test.exs

@@ -509,6 +509,77 @@ defmodule Mix.DepTest do
     end)
   end
 
+  test "nested deps with optional matching" do
+    Process.put(:custom_deps_git_repo_opts, optional: true)
+
+    # deps_repo brings git_repo but it is optional
+    deps = [
+      {:deps_repo, "0.1.0", path: "custom/deps_repo"},
+      {:git_repo, "0.1.0", git: MixTest.Case.fixture_path("git_repo")}
+    ]
+
+    with_deps(deps, fn ->
+      in_fixture("deps_status", fn ->
+        File.mkdir_p!("custom/deps_repo/lib")
+
+        File.write!("custom/deps_repo/lib/a.ex", """
+        # Check that the child dependency is top_level and optional
+        [%Mix.Dep{app: :git_repo, top_level: true, opts: opts}] = Mix.Dep.cached()
+        true = Keyword.fetch!(opts, :optional)
+        """)
+
+        Mix.Tasks.Deps.Get.run([])
+        Mix.Tasks.Deps.Compile.run([])
+      end)
+    end)
+  end
+
+  test "nested deps with runtime override on parent" do
+    Process.put(:custom_deps_git_repo_opts, runtime: false)
+
+    deps = [
+      {:deps_repo, "0.1.0", path: "custom/deps_repo"},
+      {:git_repo, "0.1.0", git: MixTest.Case.fixture_path("git_repo")}
+    ]
+
+    with_deps(deps, fn ->
+      in_fixture("deps_status", fn ->
+        File.mkdir_p!("custom/deps_repo/lib")
+
+        File.write!("custom/deps_repo/lib/a.ex", """
+        # Check that the child dependency is top_level and optional
+        [%Mix.Dep{app: :git_repo, top_level: true, opts: opts}] = Mix.Dep.cached()
+        false = Keyword.fetch!(opts, :runtime)
+        """)
+
+        Mix.Tasks.Deps.Get.run([])
+        Mix.Tasks.Deps.Compile.run([])
+      end)
+    end)
+  end
+
+  test "nested deps with runtime override on child" do
+    deps = [
+      {:deps_repo, "0.1.0", path: "custom/deps_repo"},
+      {:git_repo, "0.1.0", git: MixTest.Case.fixture_path("git_repo"), runtime: false}
+    ]
+
+    with_deps(deps, fn ->
+      in_fixture("deps_status", fn ->
+        File.mkdir_p!("custom/deps_repo/lib")
+
+        File.write!("custom/deps_repo/lib/a.ex", """
+        # Check that the child dependency is top_level and optional
+        [%Mix.Dep{app: :git_repo, top_level: true, opts: opts}] = Mix.Dep.cached()
+        false = Keyword.has_key?(opts, :runtime)
+        """)
+
+        Mix.Tasks.Deps.Get.run([])
+        Mix.Tasks.Deps.Compile.run([])
+      end)
+    end)
+  end
+
   test "nested deps with overrides" do
     # deps_repo brings git_repo but it is overriden
     deps = [