Cannot infer type argument(s) error in eclipse-SDK-I20250513-1800 (works fine in 4.35) (eclipse-jdt#4091)

Add test case passing since eclipse-jdt#4023

Fixes eclipse-jdt#4003

Author: stephan-herrmann

org.eclipse.jdt.core.tests.compiler/src/org/eclipse/jdt/core/tests/compiler/regression/GenericsRegressionTest_9.java

@@ -1192,17 +1192,17 @@ public void testGH4033() {
 			"""
 			import java.util.Collection;
 			import java.util.Iterator;
-			
+
 			public class Snippet {
 				interface Apple {}
 				interface Banana<T1, T2> {}
 				interface Smoothie<T extends Apple, M extends Apple> extends Banana<T, String> {}
-			
+
 				public static void main(String[] args) {
 					Collection<Smoothie<? extends Apple, ? extends Apple>> c = null;
 					method(c);
 				}
-			
+
 				static final <S extends Banana<? extends T, ?>, T> Iterator<T> method(
 						Collection<S> c) {
 					return null;
@@ -1235,6 +1235,63 @@ public static void main(String[] args) {
 	});
 }
 
+public void testGH4003() {
+	if (this.complianceLevel < ClassFileConstants.JDK10)
+		return; // uses 'var'
+	runConformTest(new String[] {
+		"EclipseCompilerBugReproducer.java",
+		"""
+		import java.util.List;
+		import java.util.concurrent.CompletableFuture;
+		import java.util.concurrent.CompletionStage;
+		import java.util.function.Function;
+		import java.util.function.Supplier;
+
+		public class EclipseCompilerBugReproducer {
+
+		    public static final class Mono<T> {
+
+		        public static <T> Mono<T> fromFuture(CompletionStage<? extends T> stage) {
+		            return new Mono<>();
+		        }
+
+		        public static <T> Mono<T> fromFuture(
+		                Supplier<? extends CompletionStage<? extends T>> stageSupplier) {
+		            return new Mono<>();
+		        }
+
+		        public <R> Mono<R> flatMap(Function<? super T, Mono<? extends R>> mapper) { return new Mono<>(); }
+		        public <R> Mono<R> map(Function<? super T, ? extends R> mapper)          { return new Mono<>(); }
+		        public List<T>      collectList()                                        { return List.of(); }
+		        public T            block()                                              { return null; }
+		    }
+
+		    static final class Bucket {
+		        CompletionStage<String> get(String id) {
+		            return CompletableFuture.completedFuture("value-for-" + id);
+		        }
+		    }
+
+		    /* ---------------------------------------------------------------------- */
+		    /*  Reproducer                                                            */
+		    /* ---------------------------------------------------------------------- */
+		    public static void main(String[] args) {
+
+		        var ids    = List.of("a", "b", "c");
+		        var bucket = new Bucket();
+
+		        ids.stream()
+		           .flatMap(id ->
+		               Mono.fromFuture(() -> bucket.get(id))   // <-- fails in Eclipse
+		                   .map(String::toUpperCase)
+		                   .collectList()
+		                   .stream())
+		           .forEach(System.out::println);
+		    }
+		}
+		"""
+	});
+}
 public static Class<GenericsRegressionTest_9> testClass() {
 	return GenericsRegressionTest_9.class;
 }