Fix ClassCastException in UseVarForGenericMethodInvocations (#1039)

Add instanceof check before casting to J.ParameterizedType in
makeNestedGenericsExplicit(). The code assumed all NewClass clazz
nodes are ParameterizedType, but they can also be J.Identifier
(e.g., `new String(...)`) or J.FieldAccess (e.g., `new java.lang.String(...)`).

Author: knutwannheden

src/main/java/org/openrewrite/java/migrate/lang/var/UseVarForGenericMethodInvocations.java

@@ -119,7 +119,7 @@ private J.MethodInvocation makeNestedGenericsExplicit(J.MethodInvocation mi, J.V
                 if (arg instanceof J.NewClass) {
                     J.NewClass newClass = (J.NewClass) arg;
                     // Check if using diamond operator (rightTypeParams is empty)
-                    if (!hasTypeParams(newClass.getClazz())) {
+                    if (newClass.getClazz() instanceof J.ParameterizedType && !hasTypeParams(newClass.getClazz())) {
                         // Copy type parameters from left side to right side
                         J.ParameterizedType rightType = (J.ParameterizedType) newClass.getClazz();
                         return newClass.withClazz(requireNonNull(rightType).withTypeParameters(leftTypeParams));

src/test/java/org/openrewrite/java/migrate/lang/var/UseVarForGenericMethodInvocationsTest.java

@@ -331,6 +331,70 @@ interface Path<X> { }
             );
         }
 
+        @Test
+        void nonParameterizedNewClassArgument() {
+            //language=java
+            rewriteRun(
+              version(
+                java(
+                  """
+                    import java.util.Collections;
+                    import java.util.List;
+
+                    class A {
+                      void m() {
+                          List<String> strs = Collections.singletonList(new String("test"));
+                      }
+                    }
+                    """,
+                  """
+                    import java.util.Collections;
+                    import java.util.List;
+
+                    class A {
+                      void m() {
+                          var strs = Collections.singletonList(new String("test"));
+                      }
+                    }
+                    """
+                ),
+                10
+              )
+            );
+        }
+
+        @Test
+        void fieldAccessNewClassArgument() {
+            //language=java
+            rewriteRun(
+              version(
+                java(
+                  """
+                    import java.util.Collections;
+                    import java.util.List;
+
+                    class A {
+                      void m() {
+                          List<java.lang.String> strs = Collections.singletonList(new java.lang.String("test"));
+                      }
+                    }
+                    """,
+                  """
+                    import java.util.Collections;
+                    import java.util.List;
+
+                    class A {
+                      void m() {
+                          var strs = Collections.singletonList(new java.lang.String("test"));
+                      }
+                    }
+                    """
+                ),
+                10
+              )
+            );
+        }
+
         @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/868")
         @Test
         void genericsCollectorsRegression() {