Remove unions logic while keeping the API

We keep the whole API surface intact to stay within the same major
release of structured-merge-diff even though the union code will now
panic if used in any place. Note that the schema will also allow unions
to still be specified since we want to be backwards compatible, but will
be merely ignored (which it's always been anyway).

Author: apelisse

Diff for: internal/fixture/state.go

@@ -535,8 +535,6 @@ type TestCase struct {
 	// Managed, if not nil, is the ManagedFields as expected
 	// after all operations are run.
 	Managed fieldpath.ManagedFields
-	// Set to true if the test case needs the union behavior enabled.
-	RequiresUnions bool
 	// ReportInputOnNoop if we don't want to compare the output and
 	// always return it.
 	ReturnInputOnNoop bool
@@ -576,7 +574,6 @@ func (tc TestCase) BenchWithConverter(parser Parser, converter merge.Converter)
 		Converter:         converter,
 		IgnoredFields:     tc.IgnoredFields,
 		ReturnInputOnNoop: tc.ReturnInputOnNoop,
-		EnableUnions:      tc.RequiresUnions,
 	}
 	state := State{
 		Updater: updaterBuilder.BuildUpdater(),
@@ -599,7 +596,6 @@ func (tc TestCase) TestWithConverter(parser Parser, converter merge.Converter) e
 		Converter:         converter,
 		IgnoredFields:     tc.IgnoredFields,
 		ReturnInputOnNoop: tc.ReturnInputOnNoop,
-		EnableUnions:      tc.RequiresUnions,
 	}
 	state := State{
 		Updater: updaterBuilder.BuildUpdater(),
@@ -636,15 +632,5 @@ func (tc TestCase) TestWithConverter(parser Parser, converter merge.Converter) e
 		}
 	}
 
-	if !tc.RequiresUnions {
-		// Re-run the test with unions on.
-		tc2 := tc
-		tc2.RequiresUnions = true
-		err := tc2.TestWithConverter(parser, converter)
-		if err != nil {
-			return fmt.Errorf("fails if unions are on: %v", err)
-		}
-	}
-
 	return nil
 }

Diff for: merge/union_test.go

@@ -34,6 +34,7 @@ type UpdaterBuilder struct {
 	Converter     Converter
 	IgnoredFields map[fieldpath.APIVersion]*fieldpath.Set
 
+	// Deprecated: Unions don't work, this flag will panic if used.
 	EnableUnions bool
 
 	// Stop comparing the new object with old object after applying.
@@ -46,10 +47,12 @@ type UpdaterBuilder struct {
 }
 
 func (u *UpdaterBuilder) BuildUpdater() *Updater {
+	if u.EnableUnions {
+		panic("unions don't work and have been disabled.")
+	}
 	return &Updater{
 		Converter:         u.Converter,
 		IgnoredFields:     u.IgnoredFields,
-		enableUnions:      u.EnableUnions,
 		returnInputOnNoop: u.ReturnInputOnNoop,
 	}
 }
@@ -63,17 +66,15 @@ type Updater struct {
 	// Deprecated: This will eventually become private.
 	IgnoredFields map[fieldpath.APIVersion]*fieldpath.Set
 
-	enableUnions bool
-
 	returnInputOnNoop bool
 }
 
 // EnableUnionFeature turns on union handling. It is disabled by default until the
 // feature is complete.
 //
-// Deprecated: Use the builder instead.
+// Deprecated: Unions don't work, this function will panic if used.
 func (s *Updater) EnableUnionFeature() {
-	s.enableUnions = true
+	panic("unions don't work and have been disabled.")
 }
 
 func (s *Updater) update(oldObject, newObject *typed.TypedValue, version fieldpath.APIVersion, managers fieldpath.ManagedFields, workflow string, force bool) (fieldpath.ManagedFields, *typed.Comparison, error) {
@@ -160,12 +161,6 @@ func (s *Updater) Update(liveObject, newObject *typed.TypedValue, version fieldp
 	if err != nil {
 		return nil, fieldpath.ManagedFields{}, err
 	}
-	if s.enableUnions {
-		newObject, err = liveObject.NormalizeUnions(newObject)
-		if err != nil {
-			return nil, fieldpath.ManagedFields{}, err
-		}
-	}
 	managers, compare, err := s.update(liveObject, newObject, version, managers, manager, true)
 	if err != nil {
 		return nil, fieldpath.ManagedFields{}, err
@@ -198,22 +193,10 @@ func (s *Updater) Apply(liveObject, configObject *typed.TypedValue, version fiel
 	if err != nil {
 		return nil, fieldpath.ManagedFields{}, err
 	}
-	if s.enableUnions {
-		configObject, err = configObject.NormalizeUnionsApply(configObject)
-		if err != nil {
-			return nil, fieldpath.ManagedFields{}, err
-		}
-	}
 	newObject, err := liveObject.Merge(configObject)
 	if err != nil {
 		return nil, fieldpath.ManagedFields{}, fmt.Errorf("failed to merge config: %v", err)
 	}
-	if s.enableUnions {
-		newObject, err = configObject.NormalizeUnionsApply(newObject)
-		if err != nil {
-			return nil, fieldpath.ManagedFields{}, err
-		}
-	}
 	lastSet := managers[manager]
 	set, err := configObject.ToFieldSet()
 	if err != nil {