Add clear error msg if operation not known (#5995)

Throw a clear error message if we construct a paginator spec for an
operation that is not modeled in the service model.

Author: dagnir

codegen/src/main/java/software/amazon/awssdk/codegen/poet/paginators/PaginatorsClassSpec.java

@@ -63,6 +63,9 @@ public PaginatorsClassSpec(IntermediateModel model, String c2jOperationName, Pag
         this.poetExtensions = new PoetExtension(model);
         this.typeProvider = new TypeProvider(model);
         this.operationModel = model.getOperation(c2jOperationName);
+        if (operationModel == null) {
+            throw new IllegalArgumentException("The service model does not model an operation '" + c2jOperationName + "'");
+        }
         this.paginationDocs = new PaginationDocs(model, operationModel, paginatorDefinition);
     }
 

codegen/src/test/java/software/amazon/awssdk/codegen/poet/paginators/PaginatorsClassSpecTest.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ *  http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.codegen.poet.paginators;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import com.squareup.javapoet.ClassName;
+import com.squareup.javapoet.TypeSpec;
+import org.junit.jupiter.api.Test;
+import software.amazon.awssdk.codegen.model.intermediate.IntermediateModel;
+import software.amazon.awssdk.codegen.model.service.PaginatorDefinition;
+import software.amazon.awssdk.codegen.poet.ClientTestModels;
+
+public class PaginatorsClassSpecTest {
+    @Test
+    public void constructor_unknownOperationName_throws() {
+        assertThatThrownBy(() -> new TestPaginatorSpec(ClientTestModels.awsJsonServiceModels(),
+                                                       "~~DoesNotExist",
+                                                       new PaginatorDefinition()))
+            .isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("The service model does not model an operation '~~DoesNotExist'");
+    }
+
+    private static class TestPaginatorSpec extends PaginatorsClassSpec {
+        TestPaginatorSpec(IntermediateModel model, String c2jOperationName, PaginatorDefinition paginatorDefinition) {
+            super(model, c2jOperationName, paginatorDefinition);
+        }
+
+        @Override
+        public TypeSpec poetSpec() {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public ClassName className() {
+            throw new UnsupportedOperationException();
+        }
+    }
+}