fixes #89

Author: mjgp2

src/when.js

@@ -93,7 +93,14 @@ class WhenMock {
 
           let isMatch = false
 
-          if (matchers && matchers[0] && matchers[0]._isAllArgsFunctionMatcher) {
+          if (matchers && matchers[0] &&
+              // is a possible all args matcher object
+              (typeof matchers[0] === 'function' || typeof matchers[0] === 'object') &&
+              // ensure not a proxy
+              '_isAllArgsFunctionMatcher' in matchers[0] &&
+              // check for the special property name
+              matchers[0]._isAllArgsFunctionMatcher === true
+          ) {
             if (matchers.length > 1) throw new Error('When using when.allArgs, it must be the one and only matcher provided to calledWith. You have incorrectly provided other matchers along with when.allArgs.')
             isMatch = checkArgumentMatchers(expectCall, [args])(true, matchers[0], 0)
           } else {

src/when.test.js

@@ -249,6 +249,22 @@ describe('When', () => {
         expect(fn(123, 'not a number')).toBeUndefined()
       })
 
+      it('first matcher is a proxy', () => {
+        const fn = jest.fn()
+
+        const proxy = new Proxy({}, {
+          get: () => true
+        })
+
+        when(fn)
+          .calledWith(proxy)
+          .mockReturnValue('x')
+
+        expect(proxy._isAllArgsFunctionMatcher).toEqual(true)
+
+        expect(fn(proxy)).toEqual('x')
+      })
+
       it('single arg match example', () => {
         const fn = jest.fn()
         const argAtIndex = (index, matcher) => when.allArgs((args, equals) => equals(args[index], matcher))