Missing code implementation

fixes eclipse-jdt#1755

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/problem/ProblemReporter.java

@@ -6422,6 +6422,8 @@ else if (expr instanceof CastExpression)
 			return false;
 		}
 		// fall through to bottom
+	} else if (expr instanceof LambdaExpression) {
+		// not very useful code, but humor the user
 	} else {
 		needImplementation(expr); // want to see if we get here
 		return false;

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

@@ -18737,4 +18737,31 @@ class Bar{}
 			----------
 			""");
 }
+public void testGH1755() {
+	runNegativeTest(
+		new String[] {
+			"X.java",
+			"""
+			import java.util.function.Supplier;
+
+			public class X {
+				public static void main(String[] args) {
+					String a = "Hello";
+					if (((Supplier<String>) () -> a) instanceof Supplier)
+						System.out.print("yes");
+					if (((Supplier<String>) () -> a) != null)
+						System.out.print("yes");
+				}
+			}
+			"""
+		},
+		"""
+		----------
+		1. ERROR in X.java (at line 8)
+			if (((Supplier<String>) () -> a) != null)
+			    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+		Redundant null check: this expression cannot be null
+		----------
+		""");
+}
 }