Fix identity functions for 'text-field' (i.e. 'formatted' type)

Fixes issue #7311.

src/style-spec/function/index.js

@@ -5,6 +5,7 @@ import extend from '../util/extend';
 import getType from '../util/get_type';
 import * as interpolate from '../util/interpolate';
 import Interpolate from '../expression/definitions/interpolate';
+import Formatted from '../expression/types/formatted';
 import { supportsInterpolation } from '../util/properties';
 
 export function isFunction(value) {
@@ -194,6 +195,8 @@ function evaluateExponentialFunction(parameters, propertySpec, input) {
 function evaluateIdentityFunction(parameters, propertySpec, input) {
     if (propertySpec.type === 'color') {
         input = Color.parse(input);
+    } else if (propertySpec.type === 'formatted') {
+        input = Formatted.fromString(input.toString());
     } else if (getType(input) !== propertySpec.type && (propertySpec.type !== 'enum' || !propertySpec.values[input])) {
         input = undefined;
     }

test/unit/style-spec/function.test.js

@@ -1,6 +1,7 @@
 import { test } from 'mapbox-gl-js-test';
 import { createFunction } from '../../../src/style-spec/function';
 import Color from '../../../src/style-spec/util/color';
+import Formatted from '../../../src/style-spec/expression/types/formatted';
 
 test('binary search', (t) => {
     t.test('will eventually terminate.', (t) => {
@@ -989,6 +990,22 @@ test('identity function', (t) => {
         t.end();
     });
 
+    t.test('formatted', (t) => {
+        const f = createFunction({
+            property: 'foo',
+            type: 'identity'
+        }, {
+            type: 'formatted'
+        }).evaluate;
+
+        t.deepEqual(f({zoom: 0}, {properties: {foo: 'foo'}}), Formatted.fromString('foo'));
+        t.deepEqual(f({zoom: 1}, {properties: {foo: 'bar'}}), Formatted.fromString('bar'));
+        t.deepEqual(f({zoom: 2}, {properties: {foo: 2}}), Formatted.fromString('2'));
+        t.deepEqual(f({zoom: 3}, {properties: {foo: true}}), Formatted.fromString('true'));
+
+        t.end();
+    });
+
     t.end();
 });