Provide feedback when t.is() values are deeply equal but not the same

t.is() checks if values are exactly the same, and shows a diff otherwise. However if values are deeply equal the diff is misleading. Detect this and provide appropriate feedback to the user.

Fixes #1553

lib/assert.js

@@ -87,14 +87,25 @@ function wrapAssertions(callbacks) {
 			if (Object.is(actual, expected)) {
 				pass(this);
 			} else {
-				const actualDescriptor = concordance.describe(actual, concordanceOptions);
-				const expectedDescriptor = concordance.describe(expected, concordanceOptions);
-				fail(this, new AssertionError({
-					assertion: 'is',
-					message,
-					raw: {actual, expected},
-					values: [formatDescriptorDiff(actualDescriptor, expectedDescriptor)]
-				}));
+				const result = concordance.compare(actual, expected, concordanceOptions);
+				const actualDescriptor = result.actual || concordance.describe(actual, concordanceOptions);
+				const expectedDescriptor = result.expected || concordance.describe(expected, concordanceOptions);
+
+				if (result.pass) {
+					fail(this, new AssertionError({
+						assertion: 'is',
+						message,
+						raw: {actual, expected},
+						values: [formatDescriptorWithLabel('Values are deeply equal to each other, but they are not the same:', actualDescriptor)]
+					}));
+				} else {
+					fail(this, new AssertionError({
+						assertion: 'is',
+						message,
+						raw: {actual, expected},
+						values: [formatDescriptorDiff(actualDescriptor, expectedDescriptor)]
+					}));
+				}
 			}
 		},
 

test/assert.js

@@ -213,10 +213,6 @@ test('.is()', t => {
 		assertions.is(undefined, false);
 	});
 
-	fails(t, () => {
-		assertions.is({foo: 'bar'}, {foo: 'bar'});
-	});
-
 	fails(t, () => {
 		// eslint-disable-next-line no-new-wrappers
 		assertions.is(new String('foo'), new String('foo'));
@@ -234,6 +230,19 @@ test('.is()', t => {
 		assertions.is('foo', NaN);
 	});
 
+	failsWith(t, () => {
+		assertions.is({foo: 'bar'}, {foo: 'bar'});
+	}, {
+		assertion: 'is',
+		message: '',
+		actual: {foo: 'bar'},
+		expected: {foo: 'bar'},
+		values: [{
+			label: `Values are deeply equal to each other, but they are not the same:`,
+			formatted: /foo/
+		}]
+	});
+
 	failsWith(t, () => {
 		assertions.is('foo', 'bar');
 	}, {