Make `reflection.klass` raise if `polymorphic?` not to be misused

This is an alternative of #31877 to fix #31876 caused by #28808.

This issue was caused by a combination of several loose implementation.

* finding automatic inverse association of polymorphic without context (caused by #28808)
* returning `klass` even if `polymorphic?` (exists before #28808)
* loose verification by `valid_inverse_reflection?` (exists before #28808)

This makes `klass` raise if `polymorphic?` not to be misused.
This issue will not happen unless polymorphic `klass` is misused.

Fixes #31876.
Closes #31877.

activerecord/CHANGELOG.md

 ## Rails 6.0.0.alpha (Unreleased) ##
 
+*   Make `reflection.klass` raise if `polymorphic?` not to be misused.
+
+    Fixes #31876.
+
+    *Ryuta Kamizono*
+
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

activerecord/lib/active_record/reflection.rb

@@ -416,6 +416,9 @@ def mapping
     # Active Record class.
     class AssociationReflection < MacroReflection #:nodoc:
       def compute_class(name)
+        if polymorphic?
+          raise ArgumentError, "Polymorphic association does not support to compute class."
+        end
         active_record.send(:compute_type, name)
       end
 
@@ -610,7 +613,7 @@ def automatic_inverse_of
 
             begin
               reflection = klass._reflect_on_association(inverse_name)
-            rescue NameError
+            rescue ArgumentError
               # Give up: we couldn't compute the klass type so we won't be able
               # to find any associations either.
               reflection = false

activerecord/test/cases/associations/inverse_associations_test.rb

@@ -190,6 +190,16 @@ def test_associations_with_no_inverse_of_should_return_nil
     assert_nil belongs_to_ref.inverse_of
   end
 
+  def test_polymorphic_associations_dont_attempt_to_find_inverse_of
+    belongs_to_ref = Sponsor.reflect_on_association(:sponsor)
+    assert_raise(ArgumentError) { belongs_to_ref.klass }
+    assert_nil belongs_to_ref.inverse_of
+
+    belongs_to_ref = Face.reflect_on_association(:human)
+    assert_raise(ArgumentError) { belongs_to_ref.klass }
+    assert_nil belongs_to_ref.inverse_of
+  end
+
   def test_this_inverse_stuff
     firm = Firm.create!(name: "Adequate Holdings")
     Project.create!(name: "Project 1", firm: firm)

activerecord/test/models/face.rb

@@ -2,6 +2,7 @@
 
 class Face < ActiveRecord::Base
   belongs_to :man, inverse_of: :face
+  belongs_to :human, polymorphic: true
   belongs_to :polymorphic_man, polymorphic: true, inverse_of: :polymorphic_face
   # Oracle identifier length is limited to 30 bytes or less, `polymorphic` renamed `poly`
   belongs_to :poly_man_without_inverse, polymorphic: true

activerecord/test/models/man.rb

@@ -11,3 +11,6 @@ class Man < ActiveRecord::Base
   has_many :secret_interests, class_name: "Interest", inverse_of: :secret_man
   has_one :mixed_case_monkey
 end
+
+class Human < Man
+end

activerecord/test/models/sponsor.rb

@@ -3,6 +3,7 @@
 class Sponsor < ActiveRecord::Base
   belongs_to :sponsor_club, class_name: "Club", foreign_key: "club_id"
   belongs_to :sponsorable, polymorphic: true
+  belongs_to :sponsor, polymorphic: true
   belongs_to :thing, polymorphic: true, foreign_type: :sponsorable_type, foreign_key: :sponsorable_id
   belongs_to :sponsorable_with_conditions, -> { where name: "Ernie" }, polymorphic: true,
              foreign_type: "sponsorable_type", foreign_key: "sponsorable_id"

activerecord/test/schema/schema.rb

@@ -814,6 +814,7 @@
   create_table :sponsors, force: true do |t|
     t.integer :club_id
     t.references :sponsorable, polymorphic: true, index: false
+    t.references :sponsor, polymorphic: true, index: false
   end
 
   create_table :string_key_objects, id: false, force: true do |t|
@@ -951,6 +952,7 @@
     t.string  :poly_man_without_inverse_type
     t.integer :horrible_polymorphic_man_id
     t.string  :horrible_polymorphic_man_type
+    t.references :human, polymorphic: true, index: false
   end
 
   create_table :interests, force: true do |t|