core: merge default synced folders ,don't override [GH-2873]

CHANGELOG.md

@@ -60,6 +60,8 @@ BUG FIXES:
     indefinitely. [GH-2756]
   - core: Windows checks home directory permissions more correctly to
     warn of potential issues.
+  - core: Synced folders set to the default synced folder explicitly won't
+    be deleted. [GH-2873]
   - commands/box: Box add `--force` works with `--provider` flag. [GH-2757]
   - commands/box: Listing boxes with machine-readable output crash is gone.
   - commands/plugin: Plugin installation will fail if dependencies conflict,

lib/vagrant/action/builtin/mixin_synced_folders.rb

@@ -96,7 +96,8 @@ module Vagrant
               raise Errors::NoDefaultSyncedFolderImpl, types: types
             end
 
-            folders[default_impl] = folders[""]
+            folders[default_impl] ||= {}
+            folders[default_impl].merge!(folders[""])
             folders.delete("")
           end
 

test/unit/vagrant/action/builtin/mixin_synced_folders_test.rb

@@ -75,12 +75,14 @@ describe Vagrant::Action::Builtin::MixinSyncedFolders do
     it "should return the proper set of folders" do
       folders["root"] = {}
       folders["another"] = { type: "" }
+      folders["foo"] = { type: "default" }
       folders["nfs"] = { type: "nfs" }
 
       result = subject.synced_folders(machine)
       result.length.should == 2
       result[:default].should == {
         "another" => folders["another"],
+        "foo" => folders["foo"],
         "root" => folders["root"],
       }
       result[:nfs].should == { "nfs" => folders["nfs"] }