Mutate stream returns to Stream.empty

Previously stream returns were mutated to null. This was inconsistent
and resulted in less stable mutations. Special handling was also needed
to filter out null returns used only in flatMaps, as these acted like
stream.empty.

Mutating to Stream.empty is simpler and results in more useful mutations
that are harder to detect.

pitest-entry/src/main/java/org/pitest/mutationtest/build/intercept/equivalent/EquivalentReturnMutationFilter.java

@@ -266,6 +266,7 @@ public boolean test(MutationDetails a) {
         return returnsZeroValue(method, mutatedInstruction)
             || returnsEmptyString(method, mutatedInstruction)
             || returns(method, mutatedInstruction, "java/util/Optional","empty")
+            || returns(method, mutatedInstruction, "java/util/stream/Stream","empty")
             || returns(method, mutatedInstruction, "java/util/Collections","emptyList")
             || returns(method, mutatedInstruction, "java/util/Collections","emptySet")
             || returns(method, mutatedInstruction, "java/util/List","of")

pitest-entry/src/main/resources/META-INF/services/org.pitest.mutationtest.build.MutationInterceptorFactory

@@ -17,6 +17,5 @@ org.pitest.mutationtest.build.intercept.kotlin.KotlinFilterFactory
 org.pitest.mutationtest.filter.LimitNumberOfMutationsPerClassFilterFactory
 org.pitest.mutationtest.build.intercept.equivalent.EqualsPerformanceShortcutFilterFactory
 org.pitest.mutationtest.build.intercept.equivalent.EquivalentReturnMutationFilter
-org.pitest.mutationtest.build.intercept.equivalent.NullFlatMapFilterFactory
 
 org.pitest.plugin.export.MutantExportFactory

pitest-entry/src/test/java/org/pitest/mutationtest/build/MutationDiscoveryTest.java

@@ -212,16 +212,6 @@ public void filtersEquivalentReturnValsMutants() {
     assertThat(actual).isEmpty();
   }
 
-  @Test
-  public void filtersEquivalentNullStreamMutantsUsedInFlatMapCalls() {
-    final Collection<MutationDetails> actual = findMutants(HasPrivateStreamMethodUsedOnlyInSingleFlatMap.class);
-
-    this.data.setFeatures(Collections.singletonList("-FNULLSTREAM"));
-    final Collection<MutationDetails> actualWithoutFilter = findMutants(HasPrivateStreamMethodUsedOnlyInSingleFlatMap.class);
-
-    assertThat(actual.size()).isLessThan(actualWithoutFilter.size());
-  }
-
   @Test
   public void filterMutantsInJavaRecords() {
     this.data.setDetectInlinedCode(true);

pitest-entry/src/test/java/org/pitest/mutationtest/build/intercept/equivalent/EquivalentReturnMutationFilterTest.java

@@ -13,6 +13,7 @@
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
+import java.util.stream.Stream;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.pitest.mutationtest.engine.gregor.mutators.returns.BooleanFalseReturnValsMutator.FALSE_RETURNS;
@@ -157,6 +158,11 @@ public void filtersEquivalentSetMutants() {
   public void filtersEquivalentOptionalMutants() {
     verifier.assertFiltersNMutationFromClass(1, AlreadyReturnsEmptyOptional.class);
   }
+
+  @Test
+  public void filtersEquivalentStreamMutants() {
+    verifier.assertFiltersNMutationFromClass(1, AlreadyReturnsEmptyStream.class);
+  }
 }
 
 class Widget{}
@@ -334,3 +340,9 @@ public Optional<String> a() {
     return Optional.empty();
   }
 }
+
+class AlreadyReturnsEmptyStream {
+  public Stream<String> a() {
+    return Stream.empty();
+  }
+}