Avoid infinite cycle resolving generic type that refers itself

This commit improves type resolution for a unresolved generic type that
uses itself in its upper bound declaration.

Closes gh-16451

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/TypeUtils.java

@@ -248,15 +248,28 @@ class TypeUtils {
 			TypeMirror typeMirror = descriptor.resolveGeneric(t);
 			if (typeMirror != null) {
 				if (typeMirror instanceof TypeVariable) {
-					// Still unresolved, let's use upper bound
-					return visit(((TypeVariable) typeMirror).getUpperBound(), descriptor);
+					TypeVariable typeVariable = (TypeVariable) typeMirror;
+					// Still unresolved, let's use the upper bound, checking first if
+					// a cycle may exist
+					if (!hasCycle(typeVariable)) {
+						return visit(typeVariable.getUpperBound(), descriptor);
+					}
 				}
 				else {
 					return visit(typeMirror, descriptor);
 				}
 			}
-			// Unresolved generics, use upper bound
-			return visit(t.getUpperBound(), descriptor);
+			// Fallback to simple representation of the upper bound
+			return defaultAction(t.getUpperBound(), descriptor);
+		}
+
+		private boolean hasCycle(TypeVariable variable) {
+			TypeMirror upperBound = variable.getUpperBound();
+			if (upperBound instanceof DeclaredType) {
+				return ((DeclaredType) upperBound).getTypeArguments().stream()
+						.anyMatch((candidate) -> candidate.equals(variable));
+			}
+			return false;
 		}
 
 		@Override

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessorTests.java

@@ -46,8 +46,10 @@ import org.springframework.boot.configurationsample.endpoint.SimpleEndpoint;
 import org.springframework.boot.configurationsample.endpoint.SpecificEndpoint;
 import org.springframework.boot.configurationsample.endpoint.incremental.IncrementalEndpoint;
 import org.springframework.boot.configurationsample.generic.AbstractGenericProperties;
+import org.springframework.boot.configurationsample.generic.ComplexGenericProperties;
 import org.springframework.boot.configurationsample.generic.SimpleGenericProperties;
 import org.springframework.boot.configurationsample.generic.UnresolvedGenericProperties;
+import org.springframework.boot.configurationsample.generic.UpperBoundGenericPojo;
 import org.springframework.boot.configurationsample.incremental.BarProperties;
 import org.springframework.boot.configurationsample.incremental.FooProperties;
 import org.springframework.boot.configurationsample.incremental.RenamedBarProperties;
@@ -524,6 +526,21 @@ public class ConfigurationMetadataAnnotationProcessorTests {
 		assertThat(metadata.getItems()).hasSize(3);
 	}
 
+	@Test
+	public void complexGenericProperties() {
+		ConfigurationMetadata metadata = compile(ComplexGenericProperties.class);
+		assertThat(metadata).has(
+				Metadata.withGroup("generic").fromSource(ComplexGenericProperties.class));
+		assertThat(metadata).has(
+				Metadata.withGroup("generic.test").ofType(UpperBoundGenericPojo.class)
+						.fromSource(ComplexGenericProperties.class));
+		assertThat(metadata).has(Metadata
+				.withProperty("generic.test.mappings",
+						"java.util.Map<java.lang.Enum<T>,java.lang.String>")
+				.fromSource(UpperBoundGenericPojo.class));
+		assertThat(metadata.getItems()).hasSize(3);
+	}
+
 	@Test
 	public void unresolvedGenericProperties() {
 		ConfigurationMetadata metadata = compile(AbstractGenericProperties.class,

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationsample/generic/ComplexGenericProperties.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License 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 org.springframework.boot.configurationsample.generic;
+
+import org.springframework.boot.configurationsample.ConfigurationProperties;
+import org.springframework.boot.configurationsample.NestedConfigurationProperty;
+
+/**
+ * More advanced generic setup.
+ *
+ * @author Stephane Nicoll
+ */
+@ConfigurationProperties("generic")
+public class ComplexGenericProperties {
+
+	@NestedConfigurationProperty
+	private UpperBoundGenericPojo<Test> test = new UpperBoundGenericPojo<>();
+
+	public UpperBoundGenericPojo<Test> getTest() {
+		return this.test;
+	}
+
+	public enum Test {
+
+		ONE, TWO, THREE
+
+	}
+
+}

spring-boot-project/spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationsample/generic/UpperBoundGenericPojo.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License 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 org.springframework.boot.configurationsample.generic;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A pojo with a complex generic signature.
+ *
+ * @author Stephane Nicoll
+ */
+public class UpperBoundGenericPojo<T extends Enum<T>> {
+
+	private final Map<T, String> mappings = new HashMap<>();
+
+	public Map<T, String> getMappings() {
+		return this.mappings;
+	}
+
+}