Fix double-wrapped array type for list arguments with a typed prepare method

lib/tapioca/dsl/helpers/graphql_type_helper.rb

@@ -91,15 +91,20 @@ def type_for(type, ignore_nilable_wrapper: false, prepare_method: nil)
             "T.untyped"
           end
 
+          prepared = false
           if prepare_method
             prepare_signature = Runtime::Reflection.signature_of(prepare_method)
             prepare_return_type = prepare_signature&.return_type
             if valid_return_type?(prepare_return_type)
               parsed_type = prepare_return_type&.to_s
+              prepared = true
             end
           end
 
-          if type.list?
+          # A `prepare` method receives (and returns) the argument's fully coerced value, list
+          # wrapper included, so its return type already accounts for `type.list?` and must not be
+          # wrapped again.
+          if type.list? && !prepared
             parsed_type = "T::Array[#{parsed_type}]"
           end
 

spec/tapioca/dsl/compilers/graphql_input_object_spec.rb

@@ -111,6 +111,39 @@ def post_id; end
               assert_equal(expected, rbi_for(:CreateCommentInput))
             end
 
+            it "generates correct RBI for a list argument with a prepare method returning the same list type" do
+              add_ruby_file("create_comment_input.rb", <<~RUBY)
+                class CreateCommentInput < GraphQL::Schema::InputObject
+                  extend T::Sig
+
+                  class << self
+                    extend T::Sig
+                    sig { params(tags: T::Array[String], _context: T.untyped).returns(T::Array[String]) }
+                    def prepare_tags(tags, _context)
+                      tags.map(&:downcase)
+                    end
+                  end
+
+                  argument :tags, [String], "Tags for the comment", prepare: :prepare_tags, required: true
+
+                  def resolve(tags:)
+                    # ...
+                  end
+                end
+              RUBY
+
+              expected = <<~RBI
+                # typed: strong
+
+                class CreateCommentInput
+                  sig { returns(T::Array[::String]) }
+                  def tags; end
+                end
+              RBI
+
+              assert_equal(expected, rbi_for(:CreateCommentInput))
+            end
+
             it "doesn't fail when input object is anonymous" do
               add_ruby_file("create_comment_input.rb", <<~RUBY)
                 class CreateCommentInput < GraphQL::Schema::InputObject

spec/tapioca/dsl/compilers/graphql_mutation_spec.rb

@@ -224,6 +224,77 @@ def resolve(min:, max:, other:, proc:); end
               assert_equal(expected, rbi_for(:CreateComment))
             end
 
+            it "generates correct RBI for a list argument with a prepare method returning the same list type" do
+              add_ruby_file("create_comment.rb", <<~RUBY)
+                class CreateComment < GraphQL::Schema::Mutation
+                  extend T::Sig
+
+                  class << self
+                    extend T::Sig
+                    sig { params(tags: T::Array[String], _context: T.untyped).returns(T::Array[String]) }
+                    def prepare_tags(tags, _context)
+                      tags.map(&:downcase)
+                    end
+                  end
+
+                  argument :tags, [String], "Tags for the comment", prepare: :prepare_tags
+
+                  def resolve(tags:)
+                    # ...
+                  end
+                end
+              RUBY
+
+              expected = <<~RBI
+                # typed: strong
+
+                class CreateComment
+                  sig { params(tags: T::Array[::String]).returns(T.untyped) }
+                  def resolve(tags:); end
+                end
+              RBI
+
+              assert_equal(expected, rbi_for(:CreateComment))
+            end
+
+            it "generates correct RBI for a list argument with a prepare method that has no valid return type" do
+              add_ruby_file("create_comment.rb", <<~RUBY)
+                class CreateComment < GraphQL::Schema::Mutation
+                  extend T::Sig
+
+                  class << self
+                    extend T::Sig
+                    sig { params(tags: T::Array[String], _context: T.untyped).void }
+                    def prepare_tags_void(tags, _context)
+                      tags.map(&:downcase)
+                    end
+
+                    def prepare_tags_untyped(tags, _context)
+                      tags.map(&:downcase)
+                    end
+                  end
+
+                  argument :tags, [String], "Tags for the comment", prepare: :prepare_tags_void
+                  argument :other_tags, [String], "Other tags for the comment", prepare: :prepare_tags_untyped
+
+                  def resolve(tags:, other_tags:)
+                    # ...
+                  end
+                end
+              RUBY
+
+              expected = <<~RBI
+                # typed: strong
+
+                class CreateComment
+                  sig { params(tags: T::Array[::String], other_tags: T::Array[::String]).returns(T.untyped) }
+                  def resolve(tags:, other_tags:); end
+                end
+              RBI
+
+              assert_equal(expected, rbi_for(:CreateComment))
+            end
+
             it "generates correct RBI arguments with a prepare method on the argument class" do
               add_ruby_file("create_comment.rb", <<~RUBY)
                 class CommentInput < GraphQL::Schema::InputObject