Fix HaveExactElements to work inside ContainElement or other collection matchers (#648)

Author: eiiches

matchers/have_exact_elements.go

@@ -19,6 +19,8 @@ type HaveExactElementsMatcher struct {
 }
 
 func (matcher *HaveExactElementsMatcher) Match(actual interface{}) (success bool, err error) {
+	matcher.resetState()
+
 	if isMap(actual) {
 		return false, fmt.Errorf("error")
 	}
@@ -73,3 +75,9 @@ func (matcher *HaveExactElementsMatcher) FailureMessage(actual interface{}) (mes
 func (matcher *HaveExactElementsMatcher) NegatedFailureMessage(actual interface{}) (message string) {
 	return format.Message(actual, "not to contain elements", presentable(matcher.Elements))
 }
+
+func (matcher *HaveExactElementsMatcher) resetState() {
+	matcher.mismatchFailures = nil
+	matcher.missingIndex = 0
+	matcher.extraIndex = 0
+}

matchers/have_exact_elements_test.go

@@ -110,4 +110,14 @@ to equal
 			})
 		})
 	})
+
+	When("matcher instance is reused", func() {
+		// This is a regression test for https://github.com/onsi/gomega/issues/647.
+		// Matcher instance may be reused, if placed inside ContainElement() or other collection matchers.
+		It("should work properly", func() {
+			matchSingleFalse := HaveExactElements(Equal(false))
+			Expect([]bool{true}).ShouldNot(matchSingleFalse)
+			Expect([]bool{false}).Should(matchSingleFalse)
+		})
+	})
 })