Introduce isSynthesizable in MergedAnnotation

This commit adds the ability to check if a the annotation managed by
a MergedAnnotation is synthesizable. This makes it easier to register
a JDK proxy hint if necessary

See gh-28967

spring-core/src/main/java/org/springframework/core/annotation/MergedAnnotation.java

@@ -477,6 +477,15 @@ public interface MergedAnnotation<A extends Annotation> {
 	 */
 	<T extends Map<String, Object>> T asMap(Function<MergedAnnotation<?>, T> factory, Adapt... adaptations);
 
+	/**
+	 * Determine if this merged annotation is <em>synthesizable</em>.
+	 * <p>Consult the documentation for {@link #synthesize()} for an explanation
+	 * of what is considered synthesizable.
+	 * @return {@code true} if the mapped annotation is synthesizable
+	 * @since 6.0
+	 */
+	boolean isSynthesizable();
+
 	/**
 	 * Create a type-safe synthesized version of this merged annotation that can
 	 * be used directly in code.

spring-core/src/main/java/org/springframework/core/annotation/MissingMergedAnnotation.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -134,6 +134,11 @@ final class MissingMergedAnnotation<A extends Annotation> extends AbstractMerged
 		return factory.apply(this);
 	}
 
+	@Override
+	public boolean isSynthesizable() {
+		return false;
+	}
+
 	@Override
 	public String toString() {
 		return "(missing)";

spring-core/src/main/java/org/springframework/core/annotation/TypeMappedAnnotation.java

@@ -319,6 +319,17 @@ final class TypeMappedAnnotation<A extends Annotation> extends AbstractMergedAnn
 		return value;
 	}
 
+	@Override
+	public boolean isSynthesizable() {
+		// Is this a mapped annotation for a composed annotation, and are there
+		// annotation attributes (mirrors) that need to be merged?
+		if (getDistance() > 0 && this.resolvedMirrors.length > 0) {
+			return true;
+		}
+		// Is the mapped annotation itself synthesizable?
+		return this.mapping.isSynthesizable();
+	}
+
 	@Override
 	@SuppressWarnings("unchecked")
 	protected A createSynthesizedAnnotation() {
@@ -347,22 +358,15 @@ final class TypeMappedAnnotation<A extends Annotation> extends AbstractMergedAnn
 	 * Determine if the supplied annotation has not already been synthesized
 	 * <strong>and</strong> whether the mapped annotation is a composed annotation
 	 * that needs to have its attributes merged or the mapped annotation is
-	 * {@linkplain AnnotationTypeMapping#isSynthesizable() synthesizable} in general.
+	 * {@linkplain #isSynthesizable() synthesizable} in general.
 	 * @param annotation the annotation to check
 	 * @since 5.3.22
 	 */
 	private boolean isSynthesizable(Annotation annotation) {
-		// Already synthesized?
 		if (annotation instanceof SynthesizedAnnotation) {
 			return false;
 		}
-		// Is this a mapped annotation for a composed annotation, and are there
-		// annotation attributes (mirrors) that need to be merged?
-		if (getDistance() > 0 && this.resolvedMirrors.length > 0) {
-			return true;
-		}
-		// Is the mapped annotation itself synthesizable?
-		return this.mapping.isSynthesizable();
+		return isSynthesizable();
 	}
 
 	@Override