Provide a hook point that a Configuration class was skipped [SPR-17066]

Stéphane Nicoll opened SPR-17066 and commented

Spring Boot has a ConditionEvaluationReport that keeps track of conditions evaluations but can lead to wrong information as an important bit of information is not available.

Consider the following example

@Configuration
@MyRegisterBeanCondition
@Import(Child.class)
public class Config { ... }

As the condition is in the REGISTER_BEAN phase, the configuration class will be parsed and its import directive will be applied, parsing Child in the process.

If the condition doesn't match TrackedConditionEvaluator will figure out that Config was excluded after all. As a result Child will not instantiated either. This process of figuring out that a REGISTER_BEAN condition didn't match and therefore the class shouldn't be imported is internal to the framework and there is no way for Spring Boot to know.

It would be nice to have a way to access this information. Ideally in a push model-like so that the report can update itself when the context realizes that a given class shouldn't be processed after all.

Affects: 4.3.18

Start with ConditionEvaluationReport and TrackedConditionEvaluator, using the Config, Child, and REGISTER_BEAN example in the issue to understand when imported configuration is later excluded. Determine how the framework can expose that exclusion to Spring Boot so the report can update when processing is skipped.