Apply the boxing conversion post instanceof where required (#2105)

* Fixes #2104

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/ast/InstanceOfExpression.java

@@ -128,9 +128,14 @@ public void generateCode(BlockScope currentScope, CodeStream codeStream, boolean
 			codeStream.iconst_0();
 
 		continueLabel.place();
-	} else if (!valueRequired) {
-		codeStream.pop();
 	}
+
+	if (valueRequired) {
+		codeStream.generateImplicitConversion(this.implicitConversion);
+	} else if (this.pattern == null) {
+		codeStream.pop(); // in the pattern case, if (!valueRequired), we have nothing on the stack to pop
+	}
+
 	codeStream.recordPositionsFrom(codeStream.position, this.sourceEnd);
 }
 

org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/PatternMatching16Test.java

@@ -4812,4 +4812,32 @@ public static void main(String [] args) {
 				"for\n"
 				+ "!for");
 	}
+	// https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2104
+	// [Patterns] Missing boxing conversion after instanceof leads to verify error
+	public void testBoxing() {
+		runConformTest(
+				new String[] {
+						"X.java",
+						"""
+						public class X {
+
+							public static void foo(Boolean b) {
+								System.out.println(b);
+							}
+
+							public static void main(String [] args) {
+								Object o = new Object();
+								foo(o instanceof String);
+								foo("Hello" instanceof String);
+								foo(o instanceof String s);
+								foo("Hello" instanceof String s);
+							}
+						}
+						""",
+				},
+				"false\n"
+				+ "true\n"
+				+ "false\n"
+				+ "true");
+	}
 }