Allow limited scalar inheritance

To smooth over the backwards incompatible nature of the global-ID
changes, this commit introduces a compatibilty layer, which allows
variables of type `ID` to be passed to arguments of type `GlobalID` (or
any specific sub-type).

In order for this to work, the use-site must be careful to coerce the
argument manually. The manual coercion can be removed when the
compatibilty layer is removed following the deprecation cycle.

app/graphql/types/global_id_type.rb

 # frozen_string_literal: true
 
+module GraphQLExtensions
+  module ScalarExtensions
+    # Allow ID to unify with GlobalID Types
+    def ==(other)
+      if name == 'ID' && other.is_a?(self.class) &&
+          other.type_class.ancestors.include?(::Types::GlobalIDType)
+        return true
+      end
+
+      super
+    end
+  end
+end
+
+::GraphQL::ScalarType.prepend(GraphQLExtensions::ScalarExtensions)
+
 module Types
   class GlobalIDType < BaseScalar
     graphql_name 'GlobalID'

spec/graphql/types/global_id_type_spec.rb

@@ -99,8 +99,6 @@ RSpec.describe Types::GlobalIDType do
   end
 
   describe 'compatibility' do
-    # Simplified schema to test compatibility
-
     def query(doc, vars)
       GraphQL::Query.new(schema, document: doc, context: {}, variables: vars)
     end
@@ -112,6 +110,7 @@ RSpec.describe Types::GlobalIDType do
     all_types = [::GraphQL::ID_TYPE, ::Types::GlobalIDType, ::Types::GlobalIDType[::Project]]
 
     shared_examples 'a working query' do
+      # Simplified schema to test compatibility
       let!(:schema) do
         # capture values so they can be closed over
         arg_type = argument_type
@@ -135,10 +134,21 @@ RSpec.describe Types::GlobalIDType do
               argument :id, arg_type, required: true
             end
 
+            # This is needed so that all types are always registered as input types
+            field :echo, String, null: true do
+              argument :id, ::GraphQL::ID_TYPE, required: false
+              argument :gid, ::Types::GlobalIDType, required: false
+              argument :pid, ::Types::GlobalIDType[::Project], required: false
+            end
+
             def project_by_id(id:)
               gid = ::Types::GlobalIDType[::Project].coerce_isolated_input(id)
               gid.model_class.find(gid.model_id)
             end
+
+            def echo(id: nil, gid: nil, pid: nil)
+              "id: #{id}, gid: #{gid}, pid: #{pid}"
+            end
           end)
         end
       end
@@ -163,16 +173,16 @@ RSpec.describe Types::GlobalIDType do
         GRAPHQL
       end
 
-      let(:argument_type) { ::GraphQL::ID_TYPE }
-
-      where(:result_type) { all_types }
+      where(:result_type, :argument_type) do
+        all_types.flat_map { |arg_type| all_types.zip([arg_type].cycle) }
+      end
 
       with_them do
         it_behaves_like 'a working query'
       end
     end
 
-    context 'when the argument is declared as GlobalID' do
+    context 'when the argument is passed as GlobalID' do
       let(:document) do
         <<-GRAPHQL
         query($projectId: GlobalID!) {
@@ -192,7 +202,7 @@ RSpec.describe Types::GlobalIDType do
       end
     end
 
-    context 'when the argument is declared as ProjectID' do
+    context 'when the argument is passed as ProjectID' do
       let(:document) do
         <<-GRAPHQL
         query($projectId: ProjectID!) {