NPE in TypeBinding.getSingleAbstractMethod() (eclipse-jdt#4014)

* Fixes eclipse-jdt#3956
* Fixes eclipse-jdt#4009

Author: srikanth-sankaran

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

@@ -887,7 +887,7 @@ CompatibilityResult internalIsCompatibleWith(TypeBinding targetType, Scope skope
 
 		// copy here is potentially compatible with the target type and has its shape fully computed: i.e value/void compatibility is determined and result expressions have been gathered.
 		targetType = findGroundTargetType(this.enclosingScope, targetType, targetType, argumentsTypeElided());
-		MethodBinding sam = targetType.getSingleAbstractMethod(this.enclosingScope, true);
+		MethodBinding sam = targetType == null ? null : targetType.getSingleAbstractMethod(this.enclosingScope, true);
 		if (sam == null || sam.problemId() == ProblemReasons.NoSuchSingleAbstractMethod) {
 			return CompatibilityResult.INCOMPATIBLE;
 		}

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

@@ -10473,6 +10473,95 @@ public static void main(String[] strArr) {
 			"----------\n");
 }
 
+// https://github.com/eclipse-jdt/eclipse.jdt.core/issues/3956
+// NPE in TypeBinding.getSingleAbstractMethod()
+public void testIssue3956() {
+	this.runNegativeTest(
+			new String[] {
+				"TestMe.java",
+				"""
+				import java.util.concurrent.CompletableFuture;
+
+				public class TestMe {
+					public  String id;
+					private CompletableFuture<Action> active;
+
+					public void test() {
+						CompletableFuture<Void> future;
+						if (id == null) {
+							future = active.thenAcceptAsync(recording -> {
+								recording.stop().run();
+								recording.process();
+							});
+						} else {
+							future = active.thenComposeAsync(recording -> {
+								recording.stop().run();
+				// This code (should) have compile errors but instead triggers ClassCastException
+
+								return update().handleAsync(() -> recording.process());
+
+				// Deleting the line and using this code would work
+				//				return update().handleAsync((a, b) -> {
+				//					recording.process();
+				//					return null;
+				//				});
+							});
+						}
+					}
+
+					public synchronized CompletableFuture<?> update() {
+						return null;
+					}
+
+					private static final class Action {
+
+						public Runnable stop() {
+							return () -> {
+							};
+						}
+
+						public void process() {
+
+						}
+
+					}
+				}
+				"""
+			},
+			"----------\n" +
+			"1. ERROR in TestMe.java (at line 15)\n" +
+			"	future = active.thenComposeAsync(recording -> {\n" +
+			"				recording.stop().run();\n" +
+			"// This code (should) have compile errors but instead triggers ClassCastException\n" +
+			"\n" +
+			"				return update().handleAsync(() -> recording.process());\n" +
+			"\n" +
+			"// Deleting the line and using this code would work\n" +
+			"//				return update().handleAsync((a, b) -> {\n" +
+			"//					recording.process();\n" +
+			"//					return null;\n" +
+			"//				});\n" +
+			"			});\n" +
+			"	         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n" +
+			"Type mismatch: cannot convert from CompletableFuture<Object> to CompletableFuture<Void>\n" +
+			"----------\n" +
+			"2. ERROR in TestMe.java (at line 19)\n" +
+			"	return update().handleAsync(() -> recording.process());\n" +
+			"	                ^^^^^^^^^^^\n" +
+			"The method handleAsync(BiFunction<? super capture#1-of ?,Throwable,? extends U>) in the type CompletableFuture<capture#1-of ?> is not applicable for the arguments (() -> {})\n" +
+			"----------\n" +
+			"3. ERROR in TestMe.java (at line 19)\n" +
+			"	return update().handleAsync(() -> recording.process());\n" +
+			"	                            ^^^^^^^^^^^^^^^^^^^^^^^^^\n" +
+			"Lambda expression's signature does not match the signature of the functional interface method apply(? super capture#1-of ?, Throwable)\n" +
+			"----------\n" +
+			"4. ERROR in TestMe.java (at line 19)\n" +
+			"	return update().handleAsync(() -> recording.process());\n" +
+			"	                                  ^^^^^^^^^^^^^^^^^^^\n" +
+			"Cannot return a void result\n" +
+			"----------\n");
+}
+
 public static Class testClass() {
 	return NegativeLambdaExpressionsTest.class;
 }