If AOT fails to generate the code, it doesn't provide enough context to track down the problem.

Here's an example:

Exception in thread "main" java.lang.IllegalStateException: Default code generation is not supported for bean definitions declaring an instance supplier callback: Root bean: class [null]; scope=singleton; abstract=false; lazyInit=null; autowireMode=0; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=null; factoryMethodName=null; initMethodNames=null; destroyMethodNames=null   
    at org.springframework.beans.factory.aot.DefaultBeanRegistrationCodeFragments.getTarget(DefaultBeanRegistrationCodeFragments.java:83)   
    at org.springframework.beans.factory.aot.BeanDefinitionMethodGenerator.generateBeanDefinitionMethod(BeanDefinitionMethodGenerator.java:85)  
    at org.springframework.beans.factory.aot.BeanRegistrationsAotContribution.lambda$generateRegisterBeanDefinitionsMethod$2(BeanRegistrationsAotContribution.java:90)  
    at java.base/java.util.LinkedHashMap.forEach(LinkedHashMap.java:721)    
    at org.springframework.beans.factory.aot.BeanRegistrationsAotContribution.generateRegisterBeanDefinitionsMethod(BeanRegistrationsAotContribution.java:88)   
    at org.springframework.beans.factory.aot.BeanRegistrationsAotContribution.lambda$applyTo$1(BeanRegistrationsAotContribution.java:73)    
    at org.springframework.aot.generate.GeneratedMethod.<init>(GeneratedMethod.java:54) 
    at org.springframework.aot.generate.GeneratedMethods.add(GeneratedMethods.java:112) 
    at org.springframework.aot.generate.GeneratedMethods.add(GeneratedMethods.java:89)  
    at org.springframework.beans.factory.aot.BeanRegistrationsAotContribution.applyTo(BeanRegistrationsAotContribution.java:72) 
    at org.springframework.context.aot.BeanFactoryInitializationAotContributions.applyTo(BeanFactoryInitializationAotContributions.java:78) 
    at org.springframework.context.aot.ApplicationContextAotGenerator.lambda$processAheadOfTime$0(ApplicationContextAotGenerator.java:58)   
    at org.springframework.context.aot.ApplicationContextAotGenerator.withCglibClassHandler(ApplicationContextAotGenerator.java:67) 
    at org.springframework.context.aot.ApplicationContextAotGenerator.processAheadOfTime(ApplicationContextAotGenerator.java:53)    
    at org.springframework.context.aot.ContextAotProcessor.performAotProcessing(ContextAotProcessor.java:106)   
    at org.springframework.context.aot.ContextAotProcessor.doProcess(ContextAotProcessor.java:84)   
    at org.springframework.context.aot.ContextAotProcessor.doProcess(ContextAotProcessor.java:49)   
    at org.springframework.context.aot.AbstractAotProcessor.process(AbstractAotProcessor.java:82)   
    at org.springframework.boot.SpringApplicationAotProcessor.main(SpringApplicationAotProcessor.java:80)