fix($compile): throw when non-optional '&' binding is used but not specified

Also, do not set up an expression in scope if the '&' binding is optional.

BREAKING CHANGE:

Previously, '&' expressions would always set up a function in the isolate scope. Now, if the binding
is marked as optional and the attribute is not specified, no function will be added to the isolate scope.

Finally, if the '&' expression is not optional, and the attribute is not specified, then rather than
the function being essentially a NOOP, it will instead throw an error indicating to the programmer that
a required attribute was not specified.

Closes angular#6404

Author: caitp

docs/content/error/$compile/reqattr.ngdoc

@@ -0,0 +1,9 @@
+@ngdoc error
+@name $compile:reqattr
+@fullName Required Attribute Not Present
+@description
+
+If a directive requires an attribute to be present, by specifying a non-optional isolate scope binding for that attribute, and the attribute
+not present, this error is thrown.
+
+To prevent this error from being thrown, ensure that required attributes are all specified when using a custom component.

src/ng/compile.js

@@ -1744,6 +1744,15 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                 break;
 
               case '&':
+                if (isUndefined(attrs[attrName])) {
+                  if (optional) break;
+                  isolateBindingContext[scopeName] = function unspecifiedExpressionAttr(locals) {
+                    throw $compileMinErr('reqattr',
+                        "Directive '{0}' requires attribute '{1}' which is not specified.",
+                        newIsolateScopeDirective.name, attrName);
+                  };
+                  break;
+                }
                 parentGet = $parse(attrs[attrName]);
                 isolateBindingContext[scopeName] = function(locals) {
                   return parentGet(scope, locals);

test/ng/compileSpec.js

@@ -3013,6 +3013,7 @@ describe('$compile', function() {
             optrefAlias: '=? optref',
             optreference: '=?',
             expr: '&',
+            optExpr: '&?',
             exprAlias: '&expr'
           },
           link: function(scope) {
@@ -3124,6 +3125,23 @@ describe('$compile', function() {
     }));
 
 
+    it('should throw if non-optional expression is used and not specified', inject(function($compile) {
+      compile('<div my-component></div>');
+      var isolateScope = element.isolateScope();
+      expect(typeof isolateScope.expr).toBe('function');
+      expect(function() {
+        isolateScope.expr();
+      }).toThrowMinErr('$compile', 'reqattr', "Directive 'myComponent' requires attribute 'expr' which is not specified.");
+    }));
+
+
+    it('should not create function in isolate scope if optional expression is not specified', inject(function($compile) {
+      compile('<div my-component></div>');
+      var isolateScope = element.isolateScope();
+      expect(isolateScope.optExpr).toBeUndefined();
+    }));
+
+
     describe('bind-once', function () {
 
       function countWatches(scope) {