Optional values with multiple types get coerced to an empty Array

[stanhu]

This breaks certain APIs that expect a parameter passed in with nil to remain nil. To reproduce the problem:

diff --git a/spec/grape/validations/validators/coerce_spec.rb b/spec/grape/validations/validators/coerce_spec.rb
index dbf57c9..cb2cca5 100644
--- a/spec/grape/validations/validators/coerce_spec.rb
+++ b/spec/grape/validations/validators/coerce_spec.rb
@@ -647,6 +647,19 @@ describe Grape::Validations::CoerceValidator do
         expect(last_response.body).to eq('String')
       end

+      it 'respects nil values' do
+        subject.params do
+          optional :a, types: [File, String]
+        end
+        subject.get '/' do
+          params[:a].class.to_s
+        end
+
+        get '/', a: nil
+        expect(last_response.status).to eq(200)
+        expect(last_response.body).to eq('NilClass')
+      end
+
       it 'fails when no coercion is possible' do
         subject.params do
           requires :a, types: [Boolean, Integer]

This is the test failure:

  2) Grape::Validations::CoerceValidator coerce multiple types respects nil values
     Failure/Error: expect(last_response.body).to eq('NilClass')

       expected: "NilClass"
            got: "Array"

       (compared using ==)
     # ./spec/grape/validations/validators/coerce_spec.rb:660:in `block (4 levels) in <top (required)>'

It looks like this happens because of this code block:

grape/lib/grape/validations/validators/coerce.rb

Lines 68 to 71 in 8deebb5

	# define default values for structures, the dry-types lib which is used
	# for coercion doesn't accept nil as a value, so it would fail
	if val.nil?
	return [] if type == Array || type.is_a?(Array)

I'm not quite sure I understand why the type.is_a?(Array) check needs to be there. This allows tests to pass:

$ git diff
diff --git a/lib/grape/validations/validators/coerce.rb b/lib/grape/validations/validators/coerce.rb
index 561ec1b..b2b4cbf 100644
--- a/lib/grape/validations/validators/coerce.rb
+++ b/lib/grape/validations/validators/coerce.rb
@@ -68,9 +68,10 @@ module Grape
         # define default values for structures, the dry-types lib which is used
         # for coercion doesn't accept nil as a value, so it would fail
         if val.nil?
-          return []      if type == Array || type.is_a?(Array)
+          return []      if type == Array
           return Set.new if type == Set
           return {}      if type == Hash
+          return val
         end

         converter.call(val)

Also, if we need to accept nil as a coerced value, there is https://github.com/dry-rb/dry-types/blob/bd75d556e70c2d3bef1f96140a8f33fb44c31f9d/docsite/source/optional-values.html.md.