Fix unchecked conversion not failed compilation

Detect when any explicit type argument is a raw type
Adds regression test testGH4957 reproducing the Comparator.comparing + raw Comparable scenario (issue #4957) and asserting the expected warnings/errors.
With this fix testGH4957 passes.

Author: chrisrueger

org.eclipse.jdt.core.compiler.batch/src/org/eclipse/jdt/internal/compiler/lookup/ParameterizedGenericMethodBinding.java

@@ -26,6 +26,7 @@
  *								Bug 434483 - [1.8][compiler][inference] Type inference not picked up with method reference
  *								Bug 446442 - [1.8] merge null annotations from super methods
  *								Bug 457079 - Regression: type inference
+ *     Christoph Rueger         GH4957     - ECJ compiles fine but Java does not: unchecked conversion
  *******************************************************************************/
 package org.eclipse.jdt.internal.compiler.lookup;
 
@@ -75,8 +76,17 @@ public static MethodBinding computeCompatibleMethod(MethodBinding originalMethod
 			        // incompatible due to wrong arity
 			        return new ProblemMethodBinding(originalMethod, originalMethod.selector, substitutes, ProblemReasons.TypeParameterArityMismatch);
 				}
-				methodSubstitute = environment.createParameterizedGenericMethod(originalMethod, substitutes);
-				break computeSubstitutes;
+				// JLS 15.12.2.6: Check if any explicit type argument is raw.
+				boolean usesUncheckedConversion = false;
+				for (int i = 0; i < typeVariables.length; i++) {
+					if (substitutes[i].isRawType()) {
+						usesUncheckedConversion = true;
+						break;
+					}
+				}
+				methodSubstitute = environment.createParameterizedGenericMethod(originalMethod, substitutes,
+						usesUncheckedConversion, false, null);
+			    break computeSubstitutes;
 			}
 			// perform type argument inference (15.12.2.7)
 			// initializes the map of substitutes (var --> type[][]{ equal, extends, super}

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

@@ -2295,6 +2295,68 @@ public static <T> Iterable<T> concat(Iterable<? extends Iterable<? extends T>> e
 	runner.javacTestOptions = JavacHasABug.JavacBug8016207;
 	runner.runNegativeTest();
 }
+/**
+ * https://github.com/eclipse-jdt/eclipse.jdt.core/issues/4957Eclipse
+ * 2026-03 compiles fine but Java does not: unchecked conversion with Comparator.comparing + raw Comparable
+ */
+public void testGH4957() {
+	this.runNegativeTest(
+			new String[] {
+				"Snippet.java",
+				"import java.util.Comparator;\n"
+				+ "import java.util.Map;\n"
+				+ "\n"
+				+ "public class Snippet {\n"
+				+ "\n"
+				+ "    public static void main(String[] args) {\n"
+				+ "\n"
+				+ "        Comparator<Map<String, Object>> eventComparator = Comparator\n"
+				+ "            .<Map<String, Object>, Comparable>comparing(e -> (Comparable) e.get(\"event_date\"),\n"
+				+ "                Comparator.nullsLast(Comparator.naturalOrder()))\n"
+				+ "            .thenComparing(e -> (String) e.get(\"event_type\"));\n"
+				+ "    }\n"
+				+ "\n"
+				+ "}",
+			},
+			// related to https://docs.oracle.com/javase/specs/jls/se21/html/jls-15.html#jls-15.12.2.6
+			// Eclipse was incorrectly accepting this without warnings/errors;
+		    // javac reports unchecked conversion because raw Comparable is used
+		    // and the resulting Comparator is a raw Comparator (not Comparator<Map<String,Object>>),
+		    // causing thenComparing to operate on raw type Object.
+			"----------\n" +
+			"1. WARNING in Snippet.java (at line 8)\n" +
+			"	Comparator<Map<String, Object>> eventComparator = Comparator\n" +
+			"            .<Map<String, Object>, Comparable>comparing(e -> (Comparable) e.get(\"event_date\"),\n" +
+			"                Comparator.nullsLast(Comparator.naturalOrder()))\n" +
+			"            .thenComparing(e -> (String) e.get(\"event_type\"));\n" +
+			"	                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n" +
+			"Type safety: The method thenComparing(Function) belongs to the raw type Comparator. References to generic type Comparator<T> should be parameterized\n" +
+			"----------\n" +
+			"2. WARNING in Snippet.java (at line 8)\n" +
+			"	Comparator<Map<String, Object>> eventComparator = Comparator\n" +
+			"            .<Map<String, Object>, Comparable>comparing(e -> (Comparable) e.get(\"event_date\"),\n" +
+			"                Comparator.nullsLast(Comparator.naturalOrder()))\n" +
+			"            .thenComparing(e -> (String) e.get(\"event_type\"));\n" +
+			"	                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n" +
+			"Type safety: The expression of type Comparator needs unchecked conversion to conform to Comparator<Map<String,Object>>\n" +
+			"----------\n" +
+			"3. WARNING in Snippet.java (at line 9)\n" +
+			"	.<Map<String, Object>, Comparable>comparing(e -> (Comparable) e.get(\"event_date\"),\n" +
+			"	                       ^^^^^^^^^^\n" +
+			"Comparable is a raw type. References to generic type Comparable<T> should be parameterized\n" +
+			"----------\n" +
+			"4. WARNING in Snippet.java (at line 9)\n" +
+			"	.<Map<String, Object>, Comparable>comparing(e -> (Comparable) e.get(\"event_date\"),\n" +
+			"	                                                  ^^^^^^^^^^\n" +
+			"Comparable is a raw type. References to generic type Comparable<T> should be parameterized\n" +
+			"----------\n" +
+			"5. ERROR in Snippet.java (at line 11)\n" +
+			"	.thenComparing(e -> (String) e.get(\"event_type\"));\n" +
+			"	                               ^^^\n" +
+			"The method get(String) is undefined for the type Object\n" +
+			"----------\n");
+}
+
 public static Class<GenericsRegressionTest_9> testClass() {
 	return GenericsRegressionTest_9.class;
 }