Deferred: Warn on exceptions that are likely programming errors

Fixes gh-2736
Closes gh-2737

test/unit/deferred.js

@@ -525,6 +525,65 @@ QUnit.test( "jQuery.Deferred.then - spec compatibility", function( assert ) {
 	} catch ( _ ) {}
 } );
 
+QUnit[ window.console ? "test" : "skip" ]( "jQuery.Deferred.exceptionHook", function( assert ) {
+
+	assert.expect( 1 );
+
+	var done = assert.async(),
+		defer = jQuery.Deferred(),
+		oldWarn = window.console.warn;
+
+	window.console.warn = function( msg ) {
+		assert.ok( /barf/.test( msg ), "Message: " + msg );
+	};
+	jQuery.when(
+		defer.then( function() {
+			// Should get an error
+			jQuery.barf();
+		} ).then( null, jQuery.noop ),
+		defer.then( function() {
+			// Should NOT get an error
+			throw new Error( "Make me a sandwich" );
+		} ).then( null, jQuery.noop )
+	).then( function( ) {
+		window.console.warn = oldWarn;
+		done();
+	} );
+
+	defer.resolve();
+} );
+
+QUnit[ window.console ? "test" : "skip" ]( "jQuery.Deferred.exceptionHook with stack hooks", function( assert ) {
+
+	assert.expect( 2 );
+
+	var done = assert.async(),
+		defer = jQuery.Deferred(),
+		oldWarn = window.console.warn;
+
+	jQuery.Deferred.getStackHook = function() {
+		// Default exceptionHook assumes the stack is in a form console.warn can log,
+		// but a custom getStackHook+exceptionHook pair could save a raw form and
+		// format it to a string only when an exception actually occurs.
+		// For the unit test we just ensure the plumbing works.
+		return "NO STACK FOR YOU";
+	};
+
+	window.console.warn = function( msg, stack ) {
+		assert.ok( /cough_up_hairball/.test( msg ), "Function mentioned: " + msg );
+		assert.ok( /NO STACK FOR YOU/.test( stack ), "Stack trace included: " + stack );
+	};
+	defer.then( function() {
+		jQuery.cough_up_hairball();
+	} ).then( null, function( ) {
+		window.console.warn = oldWarn;
+		delete jQuery.Deferred.getStackHook;
+		done();
+	} );
+
+	defer.resolve();
+} );
+
 QUnit.test( "jQuery.Deferred - 1.x/2.x compatibility", function( assert ) {
 
 	assert.expect( 8 );