coerce an empty string to nil in case of the bool type

Author: dnesteryuk

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 #### Fixes
 
+* [#2049](https://github.com/ruby-grape/grape/pull/2049): Coerce an empty string to nil in case of the bool type - [@dnesteryuk](https://github.com/dnesteryuk).
 * [#2043](https://github.com/ruby-grape/grape/pull/2043): Modify declared for nested array and hash - [@kadotami](https://github.com/kadotami).
 * Your contribution here.
 * [#2040](https://github.com/ruby-grape/grape/pull/2040): Fix a regression with Array of type nil - [@ericproulx](https://github.com/ericproulx).

lib/grape/validations/types/primitive_coercer.rb

@@ -36,7 +36,7 @@ def initialize(type, strict = false)
 
         def call(val)
           return InvalidValue.new if reject?(val)
-          return nil if val.nil?
+          return nil if val.nil? || treat_as_nil?(val)
           return '' if val == ''
 
           super
@@ -46,7 +46,7 @@ def call(val)
 
         attr_reader :type
 
-        # This method maintaine logic which was defined by Virtus. For example,
+        # This method maintains logic which was defined by Virtus. For example,
         # dry-types is ok to convert an array or a hash to a string, it is supported,
         # but Virtus wouldn't accept it. So, this method only exists to not introduce
         # breaking changes.
@@ -55,6 +55,13 @@ def reject?(val)
             (val.is_a?(String) && type == Hash) ||
             (val.is_a?(Hash) && type == String)
         end
+
+        # Dry-Types treats an empty string as invalid. However, Grape considers an empty string as
+        # absence of a value and coerces it into nil. See a discussion there
+        # https://github.com/ruby-grape/grape/pull/2045
+        def treat_as_nil?(val)
+          val == '' && type == Grape::API::Boolean
+        end
       end
     end
   end

spec/grape/validations/types/primitive_coercer_spec.rb

@@ -26,6 +26,10 @@
       it 'returns an error when the given value cannot be coerced' do
         expect(subject.call(123)).to be_instance_of(Grape::Validations::Types::InvalidValue)
       end
+
+      it 'coerces an empty string to nil' do
+        expect(subject.call('')).to be_nil
+      end
     end
 
     context 'String' do