Raise error when passing passing a class to :source_type

Raise `ArgumentError` when `:source_type` is given as class
in `has_many :through` relation.

Closes #41092

activerecord/lib/active_record/reflection.rb

@@ -306,6 +306,12 @@ module ActiveRecord
         def primary_key(klass)
           klass.primary_key || raise(UnknownPrimaryKey.new(klass))
         end
+
+        def ensure_option_not_given_as_class!(option_name)
+          if options[option_name] && options[option_name].class == Class
+            raise ArgumentError, "A class was passed to `:#{option_name}` but we are expecting a string."
+          end
+        end
     end
 
     # Base class for AggregateReflection and AssociationReflection. Objects of
@@ -426,9 +432,7 @@ module ActiveRecord
         @type = -(options[:foreign_type]&.to_s || "#{options[:as]}_type") if options[:as]
         @foreign_type = -(options[:foreign_type]&.to_s || "#{name}_type") if options[:polymorphic]
 
-        if options[:class_name] && options[:class_name].class == Class
+        ensure_option_not_given_as_class!(:class_name)
-          raise ArgumentError, "A class was passed to `:class_name` but we are expecting a string."
-        end
       end
 
       def association_scope_cache(klass, owner, &block)
@@ -748,6 +752,8 @@ module ActiveRecord
         @delegate_reflection = delegate_reflection
         @klass = delegate_reflection.options[:anonymous_class]
         @source_reflection_name = delegate_reflection.options[:source]
+
+        ensure_option_not_given_as_class!(:source_type)
       end
 
       def through_reflection?

activerecord/test/cases/reflection_test.rb

@@ -432,6 +432,13 @@ class ReflectionTest < ActiveRecord::TestCase
     assert_equal "A class was passed to `:class_name` but we are expecting a string.", error.message
   end
 
+  def test_class_for_source_type
+    error = assert_raises(ArgumentError) do
+      ActiveRecord::Reflection.create(:has_many, :tagged_posts, nil, { through: :taggings, source: :taggable, source_type: Post }, Tag)
+    end
+    assert_equal "A class was passed to `:source_type` but we are expecting a string.", error.message
+  end
+
   def test_join_table
     category = Struct.new(:table_name, :pluralize_table_names).new("categories", true)
     product = Struct.new(:table_name, :pluralize_table_names).new("products", true)