Fix validation of underscored names

Author: nepalez

lib/dry/initializer/dispatchers/build_nested_type.rb

@@ -13,7 +13,7 @@ module Dry::Initializer::Dispatchers::BuildNestedType
   # rubocop: disable Metrics/ParameterLists
   def call(parent:, source:, target:, type: nil, block: nil, **options)
     check_certainty!(source, type, block)
-    check_name!(target)
+    check_name!(target, block)
     type ||= build_nested_type(parent, target, block)
     { parent: parent, source: source, target: target, type: type, **options }
   end
@@ -22,16 +22,17 @@ def call(parent:, source:, target:, type: nil, block: nil, **options)
   private
 
   def check_certainty!(source, type, block)
-    return unless type
     return unless block
+    return unless type
 
     raise ArgumentError, <<~MESSAGE
       You should define coercer of values of argument '#{source}'
       either though the parameter/option, or via nested block, but not the both.
     MESSAGE
   end
 
-  def check_name!(name)
+  def check_name!(name, block)
+    return unless block
     return unless name[/^_|__|_$/]
 
     raise ArgumentError, <<~MESSAGE

spec/type_constraint_spec.rb

@@ -5,14 +5,14 @@
     before do
       class Test::Foo
         extend Dry::Initializer
-        param :foo, proc(&:to_s), optional: true
+        param :__foo__, proc(&:to_s), optional: true
       end
     end
 
     subject { Test::Foo.new :foo }
 
     it "coerces a value" do
-      expect(subject.foo).to eq "foo"
+      expect(subject.__foo__).to eq "foo"
     end
   end