allow users to wrap the Gomega interface (#521)

gomega internally casts to *internal.Gomega which makes wrapping
it impossible.  To allow this, add an interface that if implemented
will be used to get the actual *internal.Gomega.  This allows users
to replace gomega.Default with their own wrapper as long as it
implements provides an 'Inner() Gomega' method.

gomega_dsl.go

@@ -52,7 +52,7 @@ var Default = Gomega(internal.NewGomega(internal.FetchDefaultDurationBundle()))
 // rich ecosystem of matchers without causing a test to fail.  For example, to aggregate a series of potential failures
 // or for use in a non-test setting.
 func NewGomega(fail types.GomegaFailHandler) Gomega {
-	return internal.NewGomega(Default.(*internal.Gomega).DurationBundle).ConfigureWithFailHandler(fail)
+	return internal.NewGomega(internalGomega(Default).DurationBundle).ConfigureWithFailHandler(fail)
 }
 
 // WithT wraps a *testing.T and provides `Expect`, `Eventually`, and `Consistently` methods.  This allows you to leverage
@@ -69,6 +69,20 @@ type WithT = internal.Gomega
 // GomegaWithT is deprecated in favor of gomega.WithT, which does not stutter.
 type GomegaWithT = WithT
 
+// inner is an interface that allows users to provide a wrapper around Default.  The wrapper
+// must implement the inner interface and return either the original Default or the result of
+// a call to NewGomega().
+type inner interface {
+	Inner() Gomega
+}
+
+func internalGomega(g Gomega) *internal.Gomega {
+	if v, ok := g.(inner); ok {
+		return v.Inner().(*internal.Gomega)
+	}
+	return g.(*internal.Gomega)
+}
+
 // NewWithT takes a *testing.T and returngs a `gomega.WithT` allowing you to use `Expect`, `Eventually`, and `Consistently` along with
 // Gomega's rich ecosystem of matchers in standard `testing` test suits.
 //
@@ -79,7 +93,7 @@ type GomegaWithT = WithT
 //        g.Expect(f.HasCow()).To(BeTrue(), "Farm should have cow")
 //     }
 func NewWithT(t types.GomegaTestingT) *WithT {
-	return internal.NewGomega(Default.(*internal.Gomega).DurationBundle).ConfigureWithT(t)
+	return internal.NewGomega(internalGomega(Default).DurationBundle).ConfigureWithT(t)
 }
 
 // NewGomegaWithT is deprecated in favor of gomega.NewWithT, which does not stutter.
@@ -88,20 +102,20 @@ var NewGomegaWithT = NewWithT
 // RegisterFailHandler connects Ginkgo to Gomega. When a matcher fails
 // the fail handler passed into RegisterFailHandler is called.
 func RegisterFailHandler(fail types.GomegaFailHandler) {
-	Default.(*internal.Gomega).ConfigureWithFailHandler(fail)
+	internalGomega(Default).ConfigureWithFailHandler(fail)
 }
 
 // RegisterFailHandlerWithT is deprecated and will be removed in a future release.
 // users should use RegisterFailHandler, or RegisterTestingT
 func RegisterFailHandlerWithT(_ types.GomegaTestingT, fail types.GomegaFailHandler) {
 	fmt.Println("RegisterFailHandlerWithT is deprecated.  Please use RegisterFailHandler or RegisterTestingT instead.")
-	Default.(*internal.Gomega).ConfigureWithFailHandler(fail)
+	internalGomega(Default).ConfigureWithFailHandler(fail)
 }
 
 // RegisterTestingT connects Gomega to Golang's XUnit style
 // Testing.T tests.  It is now deprecated and you should use NewWithT() instead to get a fresh instance of Gomega for each test.
 func RegisterTestingT(t types.GomegaTestingT) {
-	Default.(*internal.Gomega).ConfigureWithT(t)
+	internalGomega(Default).ConfigureWithT(t)
 }
 
 // InterceptGomegaFailures runs a given callback and returns an array of
@@ -112,13 +126,13 @@ func RegisterTestingT(t types.GomegaTestingT) {
 // This is most useful when testing custom matchers, but can also be used to check
 // on a value using a Gomega assertion without causing a test failure.
 func InterceptGomegaFailures(f func()) []string {
-	originalHandler := Default.(*internal.Gomega).Fail
+	originalHandler := internalGomega(Default).Fail
 	failures := []string{}
-	Default.(*internal.Gomega).Fail = func(message string, callerSkip ...int) {
+	internalGomega(Default).Fail = func(message string, callerSkip ...int) {
 		failures = append(failures, message)
 	}
 	defer func() {
-		Default.(*internal.Gomega).Fail = originalHandler
+		internalGomega(Default).Fail = originalHandler
 	}()
 	f()
 	return failures
@@ -131,14 +145,14 @@ func InterceptGomegaFailures(f func()) []string {
 // does not register a failure with the FailHandler registered via RegisterFailHandler - it is up
 // to the user to decide what to do with the returned error
 func InterceptGomegaFailure(f func()) (err error) {
-	originalHandler := Default.(*internal.Gomega).Fail
-	Default.(*internal.Gomega).Fail = func(message string, callerSkip ...int) {
+	originalHandler := internalGomega(Default).Fail
+	internalGomega(Default).Fail = func(message string, callerSkip ...int) {
 		err = errors.New(message)
 		panic("stop execution")
 	}
 
 	defer func() {
-		Default.(*internal.Gomega).Fail = originalHandler
+		internalGomega(Default).Fail = originalHandler
 		if e := recover(); e != nil {
 			if err == nil {
 				panic(e)
@@ -151,7 +165,7 @@ func InterceptGomegaFailure(f func()) (err error) {
 }
 
 func ensureDefaultGomegaIsConfigured() {
-	if !Default.(*internal.Gomega).IsConfigured() {
+	if !internalGomega(Default).IsConfigured() {
 		panic(nilGomegaPanic)
 	}
 }