Fix nullability of GraphQL scalar types when passed as required arguments

lib/tapioca/dsl/helpers/graphql_type_helper.rb

@@ -80,7 +80,9 @@ def type_for(type, ignore_nilable_wrapper: false, prepare_method: nil)
             signature = Runtime::Reflection.signature_of(method)
             return_type = signature&.return_type
 
-            valid_return_type?(return_type) ? return_type.to_s : "T.untyped"
+            # Wrap as non-nilable for required arguments. `coerce_input` supports both
+            # required and optional; optional arguments are re-wrapped below based on `type.non_null?`
+            valid_return_type?(return_type) ? RBIHelper.as_non_nilable_type(return_type.to_s) : "T.untyped"
           when GraphQL::Schema::InputObject.singleton_class
             type_for_constant(unwrapped_type)
           when Module

spec/tapioca/dsl/compilers/graphql_mutation_spec.rb

@@ -361,6 +361,45 @@ def resolve(loaded_argument:, loaded_arguments:, custom_name:, optional_loaded_a
               assert_equal(expected, rbi_for(:CreateComment))
             end
 
+            it "generates correct RBI for custom scalars whose coerce_input returns a nilable type" do
+              add_ruby_file("create_comment.rb", <<~RUBY)
+                class CustomScalar; end
+
+                class NilableScalarType < GraphQL::Schema::Scalar
+                  class << self
+                    extend T::Sig
+
+                    sig { params(value: T.untyped, context: GraphQL::Query::Context).returns(T.nilable(CustomScalar)) }
+                    def coerce_input(value, context)
+                      return nil if value.nil?
+                      CustomScalar.new
+                    end
+                  end
+                end
+
+                class CreateComment < GraphQL::Schema::Mutation
+                  argument :required_scalar, NilableScalarType, required: true
+                  argument :required_scalar_array, [NilableScalarType], required: true
+                  argument :optional_scalar, NilableScalarType, required: false
+
+                  def resolve(required_scalar:, required_scalar_array:, optional_scalar: nil)
+                    # ...
+                  end
+                end
+              RUBY
+
+              expected = <<~RBI
+                # typed: strong
+
+                class CreateComment
+                  sig { params(required_scalar: ::CustomScalar, required_scalar_array: T::Array[::CustomScalar], optional_scalar: T.nilable(::CustomScalar)).returns(T.untyped) }
+                  def resolve(required_scalar:, required_scalar_array:, optional_scalar: T.unsafe(nil)); end
+                end
+              RBI
+
+              assert_equal(expected, rbi_for(:CreateComment))
+            end
+
             it "generates correct RBI for custom scalars with return types" do
               add_ruby_file("create_comment.rb", <<~RUBY)
                 class CustomScalar; end