Deferred: Don't warn on setting getStackHook to the getErrorHook value

Don't warn if `jQuery.Deferred.getStackHook` is set to the value of
`jQuery.Deferred.getErrorHook`. This is to facilitate plugins supporting both
jQuery <3.7 and older without triggering Migrate warnings and without requiring
complex logic parsing `jQuery.fn.jquery`.

Closes gh-577
Ref gh-578

Author: mgol

src/jquery/deferred.js

@@ -48,9 +48,13 @@ Object.defineProperty( jQuery.Deferred, "getStackHook", {
 	},
 	set: function( newValue ) {
 		if ( jQuery.migrateIsPatchEnabled( "deferred-getStackHook" ) ) {
-			migrateWarn( "deferred-getStackHook",
-				"jQuery.Deferred.getStackHook is removed; use jQuery.Deferred.getErrorHook" );
-			jQuery.Deferred.getErrorHook = newValue;
+
+			// Only warn if `getErrorHook` wasn't set to the same value first.
+			if ( jQuery.Deferred.getErrorHook !== newValue ) {
+				migrateWarn( "deferred-getStackHook",
+					"jQuery.Deferred.getStackHook is removed; use jQuery.Deferred.getErrorHook" );
+				jQuery.Deferred.getErrorHook = newValue;
+			}
 		} else {
 			unpatchedGetStackHookValue = newValue;
 		}

test/unit/jquery/deferred.js

@@ -76,7 +76,7 @@ QUnit.test( "jQuery.Deferred.getStackHook - getter, no getErrorHook", function(
 } );
 
 QUnit.test( "jQuery.Deferred.getStackHook - setter", function( assert ) {
-	assert.expect( 5 );
+	assert.expect( 6 );
 
 	var exceptionHookSpy,
 		done = assert.async();
@@ -90,6 +90,11 @@ QUnit.test( "jQuery.Deferred.getStackHook - setter", function( assert ) {
 			"getStackHook mirrors getErrorHook (setter)" );
 	} );
 
+	expectNoMessage( assert, "jQuery.Deferred.getStackHook - setter", 1, function() {
+		var mockFn = function() {};
+		jQuery.Deferred.getStackHook = jQuery.Deferred.getErrorHook = mockFn;
+	} );
+
 	expectMessage( assert, "asyncHook from jQuery.Deferred.getStackHook reported",
 			1, function() {
 		jQuery.Deferred.getStackHook = function() {

warnings.md

@@ -161,15 +161,15 @@ This is _not_ a warning, but a console log message the plugin shows when it firs
 
 ### \[jsonp-promotion\] JQMIGRATE: JSON-to-JSONP auto-promotion is removed
 
-**Cause:** `jQuery.ajax` calls with `dataType: 'json'` with a provided callback are automatically converted by jQuery to JSONP requests unless the options also specify `jsonp: false`. Auto-promoting JSON requests to JSONP introduces a security risk as the developer may be unaware they're not just downloading data but executing code from a remote domain. This auto-promoting behavior is deprecated and will be removed in jQuery 4.0.0.
+**Cause:** `jQuery.ajax` calls with `dataType: 'json'` with a provided callback are automatically converted by jQuery to JSONP requests unless the options also specify `jsonp: false`. Auto-promoting JSON requests to JSONP introduces a security risk as the developer may be unaware they're not just downloading data but executing code from a remote domain. This auto-promoting behavior has been removed in jQuery 4.0.0.
 
 **Solution:** To trigger a JSONP request, specify the `dataType: "jsonp"` option.
 
 ### \[deferred-getStackHook\] JQMIGRATE: jQuery.Deferred.getStackHook is removed; use jQuery.Deferred.getErrorHook
 
-**Cause:** `jQuery.Deferred.getStackHook` was originally created to pass the stack trace from before an async barrier to report when a user error (like calling a non-existing function) causes a promise to be rejected. However, passing a stack trace doesn't take source maps into account, so we started advising to pass the whole error object. To make it clearer, we also renamed the API to `jQuery.Deferred.getErrorHook`. The legacy alias will be removed in jQuery 4.0.0
+**Cause:** `jQuery.Deferred.getStackHook` was originally created to pass the stack trace from before an async barrier to report when a user error (like calling a non-existing function) causes a promise to be rejected. However, passing a stack trace doesn't take source maps into account, so we started advising to pass the whole error object. To make it clearer, we also renamed the API to `jQuery.Deferred.getErrorHook`. The legacy alias has been removed in jQuery 4.0.0.
 
-**Solution:** Rename all usage of `jQuery.Deferred.getStackHook` to `jQuery.Deferred.getErrorHook`. If you previously assigned a function returning an error stack to `jQuery.Deferred.getStackHook` or `jQuery.Deferred.getErrorHook`, change it to return a full error object.
+**Solution:** Rename all usage of `jQuery.Deferred.getStackHook` to `jQuery.Deferred.getErrorHook`. If you previously assigned a function returning an error stack to `jQuery.Deferred.getStackHook` or `jQuery.Deferred.getErrorHook`, change it to return a full error object. If you aim to still support jQuery <3.7, assign the hook to `jQuery.Deferred.getErrorHook` first and only later to `jQuery.Deferred.getStackHook` to avoid a Migrate warning.
 
 
 ## Deprecated APIs