[java] UnnecessaryImport - keep analyzing with failed overload selection

Fixes pmd#4816

docs/pages/release_notes.md

@@ -284,6 +284,7 @@ The rules have been moved into categories with PMD 6.
   * [#4631](https://github.com/pmd/pmd/issues/4631): \[java] UnnecessaryFullyQualifiedName fails to recognize illegal self reference in enums
   * [#4645](https://github.com/pmd/pmd/issues/4645): \[java] CommentDefaultAccessModifier - False Positive with JUnit5's ParameterizedTest
   * [#4754](https://github.com/pmd/pmd/pull/4754):   \[java] EmptyControlStatementRule: Add allowCommentedBlocks property
+  * [#4816](https://github.com/pmd/pmd/issues/4816): \[java] UnnecessaryImport false-positive on generic method call with on lambda
 * java-design
   * [#174](https://github.com/pmd/pmd/issues/174):   \[java] SingularField false positive with switch in method that both assigns and reads field
 * java-errorprone
@@ -1442,6 +1443,7 @@ Language specific fixes:
     * [#4631](https://github.com/pmd/pmd/issues/4631): \[java] UnnecessaryFullyQualifiedName fails to recognize illegal self reference in enums
     * [#4645](https://github.com/pmd/pmd/issues/4645): \[java] CommentDefaultAccessModifier - False Positive with JUnit5's ParameterizedTest
     * [#4754](https://github.com/pmd/pmd/pull/4754):   \[java] EmptyControlStatementRule: Add allowCommentedBlocks property
+    * [#4816](https://github.com/pmd/pmd/issues/4816): \[java] UnnecessaryImport false-positive on generic method call with on lambda
 * java-design
     * [#174](https://github.com/pmd/pmd/issues/174):   \[java] SingularField false positive with switch in method that both assigns and reads field
     * [#1014](https://github.com/pmd/pmd/issues/1014): \[java] LawOfDemeter: False positive with lambda expression

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/codestyle/UnnecessaryImportRule.java

@@ -235,7 +235,8 @@ public Object visit(ASTMethodCall node, Object data) {
         if (node.getQualifier() == null) {
             OverloadSelectionResult overload = node.getOverloadSelectionInfo();
             if (overload.isFailed()) {
-                return null; // todo we're erring towards FPs
+                // don't try further, but still visit all ASTClassType nodes in the AST.
+                return super.visit(node, data); // todo we're erring towards FPs
             }
 
             ShadowChainIterator<JMethodSig, ScopeInfo> scopeIter =

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/rule/codestyle/unnecessaryimport/item/Item.java

@@ -0,0 +1,11 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport.item;
+
+public class Item {
+    public String getValue() {
+        return "";
+    }
+}

pmd-java/src/test/java/net/sourceforge/pmd/lang/java/rule/codestyle/unnecessaryimport/item/ItemProducer.java

@@ -0,0 +1,13 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport.item;
+
+import java.util.stream.Stream;
+
+public class ItemProducer {
+    public Stream<Item> stream() {
+        return null;
+    }
+}

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/codestyle/xml/UnnecessaryImport.xml

@@ -1168,4 +1168,31 @@ public class Sample {
             }
             ]]></code>
     </test-code>
+
+    <test-code>
+        <description>[java] UnnecessaryImport false-positive on generic method call with on lambda #4816</description>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+package net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport;
+
+import java.util.function.Function;
+import java.util.stream.Collectors; // UnnecessaryImport:	Unused import 'java.util.stream.Collectors'
+import java.util.TreeSet; // UnnecessaryImport:	Unused import 'java.util.TreeSet'
+import java.util.Set;
+
+import net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport.item.Item; // UnnecessaryImport:	Unused import 'net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport.item.Item'
+import net.sourceforge.pmd.lang.java.rule.codestyle.unnecessaryimport.item.ItemProducer;
+
+public class Example {
+    private <X> X run(Function<ItemProducer, X> f) { return null; }
+
+    public Set<String> sample() {
+        return run((producer) -> producer
+            .stream()
+            .map(Item::getValue)
+            .collect(Collectors.toCollection(TreeSet::new)));
+    }
+}
+]]></code>
+    </test-code>
 </test-data>