Fix UnusedAssignment false positive with unconditional if(true) overwrite

- Fix the false positive in DataflowPass where assignments in unconditional
  if(true) blocks were not properly marked as overwriting previous assignments
- Update test expectations to reflect the fix
- Addresses issue pmd#6518

Author: leemeii

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/internal/DataflowPass.java

@@ -481,17 +481,30 @@ public SpanInfo visit(ASTConditionalExpression node, SpanInfo data) {
             return makeConditional(data, node.getCondition(), node.getThenBranch(), node.getElseBranch());
         }
 
-        SpanInfo makeConditional(SpanInfo before, ASTExpression condition, JavaNode thenBranch, JavaNode elseBranch) {
-            SpanInfo thenState = before.fork();
-            SpanInfo elseState = elseBranch != null ? before.fork() : before;
+    SpanInfo makeConditional(SpanInfo before, ASTExpression condition, JavaNode thenBranch, JavaNode elseBranch) {
+        // Handle literal boolean conditions specially
+        if (JavaAstUtils.isBooleanLiteral(condition, true)) {
+            // if (true): evaluate only condition and then-branch
+            SpanInfo state = acceptOpt(condition, before);
+            state = acceptOpt(thenBranch, state);
+            return state;
+        } else if (JavaAstUtils.isBooleanLiteral(condition, false)) {
+            // if (false): evaluate only condition and else-branch
+            SpanInfo state = acceptOpt(condition, before);
+            state = acceptOpt(elseBranch, state);
+            return state;
+        }
+        
+        SpanInfo thenState = before.fork();
+        SpanInfo elseState = elseBranch != null ? before.fork() : before;
 
-            linkConditional(before, condition, thenState, elseState, true);
+        linkConditional(before, condition, thenState, elseState, true);
 
-            thenState = acceptOpt(thenBranch, thenState);
-            elseState = acceptOpt(elseBranch, elseState);
+        thenState = acceptOpt(thenBranch, thenState);
+        elseState = acceptOpt(elseBranch, elseState);
 
-            return elseState.absorb(thenState);
-        }
+        return elseState.absorb(thenState);
+    }
 
         /*
          * This recursive procedure translates shortcut conditionals

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/bestpractices/xml/UnusedAssignment.xml

@@ -4020,4 +4020,88 @@ class Lock {
             }
             ]]></code>
     </test-code>
+
+    <test-code>
+        <description>[java] UnusedAssignment: fix false negative with unconditional if(true) overwrite #6518</description>
+        <expected-problems>1</expected-problems>
+        <expected-linenumbers>3</expected-linenumbers>
+        <expected-messages>
+            <message>The initializer for variable 'x' is never used (overwritten on line 5)</message>
+        </expected-messages>
+        <code><![CDATA[
+public class Test {
+    void test() {
+        int x = 0; // should be reported as unused
+        if (true) {
+            x = 1;
+        }
+        System.out.println(x);
+    }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>[java] UnusedAssignment: if(false) case</description>
+        <expected-problems>1</expected-problems>
+        <expected-linenumbers>3</expected-linenumbers>
+        <expected-messages>
+            <message>The initializer for variable 'x' is never used</message>
+        </expected-messages>
+        <code><![CDATA[
+public class Test {
+    void test() {
+        int x = 0; // should be reported as unused
+        if (false) {
+            x = 1;
+        }
+        System.out.println(x);
+    }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>[java] UnusedAssignment: if(true) with else branch</description>
+        <expected-problems>1</expected-problems>
+        <expected-linenumbers>3</expected-linenumbers>
+        <expected-messages>
+            <message>The initializer for variable 'x' is never used (overwritten on line 5)</message>
+        </expected-messages>
+        <code><![CDATA[
+public class Test {
+    void test() {
+        int x = 0; // should be reported as unused
+        if (true) {
+            x = 1;
+        } else {
+            x = 2;
+        }
+        System.out.println(x);
+    }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>[java] UnusedAssignment: if(false) with else branch</description>
+        <expected-problems>1</expected-problems>
+        <expected-linenumbers>3</expected-linenumbers>
+        <expected-messages>
+            <message>The initializer for variable 'x' is never used (overwritten on line 7)</message>
+        </expected-messages>
+        <code><![CDATA[
+public class Test {
+    void test() {
+        int x = 0; // should be reported as unused
+        if (false) {
+            x = 1;
+        } else {
+            x = 2;
+        }
+        System.out.println(x);
+    }
+}
+        ]]></code>
+    </test-code>
 </test-data>